What you say about print size, resolution, viewing distance is correct, however in practice very few ‘viewers’ do that . Whatever the size of the image people tend to view it Much closer than the distance you suggest.
And many gallery spaces could not accommodate those distances.
So to look their best larger prints can benefit from additional ‘dpi’ instead of 360 dpi some folks go to 720 on Epson large format printers ( those numbers are multiples of the number of nozzles in the Epson print head ( 300/600 for Canon & others
Most top printers are using software RIP’s like Color Byte’s Image print which replaces the Epson driver completely, does extensive layout, and automatic re-sizing
Buy frankly I have found that with Image Print 360 works great and 720 is more of a special case. But I Have seen it work
( I think Lloyd Chambers @diglloyd has done ‘super resolution printing demos but I’m not positive)
Re seeing differences, it is difficult to see subtlety on the internet.

Nice job! Your bracket looks large enough so that the camera can rotate around the optical center. I love L brackets

Not sure why this is 'news' ? Anyone who has ever emailed a large image as a small attachment knows there are compromises. Sometimes severe.
If the Goal is a Print, the best large format pigment printers can print larger gamuts than your display (when sent as a 16b TIFF) which is good, but the display has a larger dynamic range than a paper print. So 'differences' abound in the process, controlling them for artistic intent is important

I think 'A.I.' is a losing (or winning) battle, depending on your point of View.
Linda from Maine has been articulate about the futility of "drawing lines"
AI is in its infancy, it will only get more powerful and "Harder to detect" (if that is your aim)
As technology advances, what is truly under our control tends to diminish.
The ubiquity of AI makes it increasingly difficult to avoid, even if you want to.
If you learned with film, and define that as relatively pure (because even manipulations were done by the photographer) To a large extent, That ship has sailed.
If you edit RAW files, most editing applications will increasingly include machine learning to some degree.
If you "Replace skies" the reality of the capture is gone.
Like many, I tend to think of myself as a relative purist. But, also like many, I love having an iPhone in my pocket, and 'take credit' for the good pictures, when the truth is none of them are entirely my doing. At best I can take credit for the timing of the snap, and the general direction in which the phone was pointed.
Even our cameras sometimes do lens corrections without consulting us.
So, for Me & my work, about the best I can do, is to state upfront what I do, and what I don't do, and then make an honest attempt to stick to that.
But even that approach is not sustainable, because we will each have a different set of things we do & don't do---even in the big things. The nuances, I suspect are a lost cause.
It would seem to me that the most rational approach is to evolve with technology while attempting to create what we believe to be honest work.
I think the OP was correct is raising the question, we should all think about it, but there won't be a single answer.
At least that is where I am now.
The other issue is that I must confess is that I feel a bit less motivated to go out & make landscapes the "Old fashioned way" because the expectation seems to be "These may not be real" I have been turning more towards pictures of people because the reality is easier to confirm.
I don't think any of what I say here is necessarily "correct' but just an honest shot at a thoughtful response to rapidly advancing technology.
Creative people will always use whatever technology is available, and any new technology gets over used and abused on its way to honest work. (Think Fisheye lenses)
These are growing pains.

There is no free lunch, B&H taking on all the sales tax for PAYBOO sales has to come from somewhere.
So they picked a bank that charges the maximum legal interest rate and get a cut from the bank to py the tax, which has to be paid at the time of sale.
You need to read the terms & conditions before you agree. Waiting until AFTER it is done, benefits no one except the bank.
If the interest rate number reported above is accurate, You agreed to pay over $1800 bucks a year on a $6200 Purchase

If you exercise reasonable caution, you will not have a problem.
The 200-600 has a lens foot. Use it for both tripod mounting and strap mounting.
This way there is never any significant stress on the lens mount.
Something like a black rapid cross body strap and an Acratech Swift clamp, is MADE for this exact purpose.
It is not the only one on the market, but the best engineered one. It has variable tension on the clamp (to accommodate out of spec Arca plates), a safety on the lever release, AND a brass pin that stops any possibility of the plate sliding out.
For lenses without tripod feet, an Arca bottom plate or L-bracket spreads the load a bit, and the Swift clamp solves the strap issue there too.

I sometimes dream about being rich enough to have someone make me an 8x10 sensor, because the LOOK of those long lenses and their ability to draw an image at scale is something I do miss. But then I would soon begin complaining about carrying the thing..... Digital is just SO convenient, and amazingly good,...........
The Problem is, I KNOW I'm a hypocrite, I could order some 8x10 Tri-X and just do it, make contact prints or scan it.
I have all the stuff.... the spirit is willing, but the Knees are weak......

There are analog revivals everywhere, Photography, Music, woodworking with hand tools and lots of other crafts.
People miss the involvement, producing work with Heart head and hands. There is participation in the process and a deep sense of involvement. It is a real thing, and in some ways a re-discovery, in other ways a respite from the unseen digital world.
It is not necessarily about better or worse, nor is it a competition.
There IS an aspect of analog being 'trendy', which is OK but I am more talking about people pursuing it for deeper reasons.... the physical connection to the production of a creative or useful thing.
Most of my Photographic life was analog, (probably like many here) I still own view cameras from 4x5 to 11x14"
I miss the tactile nature of it, but I am one who has moved to digital because it is far easier, and in most objective respects Better. But I am also a woodworker , gardener etc . We send our grand children to schools where every kid plays a musical instrument, they do gardening, painting and drawing as part of the curriculum.
They also participate in the digital world, but with balance.
I love technology, it has made all our lives better, we fully participate, but also enforce a balance.
So I understand the affection for, and attraction to the analog world .....it's all good.......

.....So what was that combination..... 2x x 560mm 1120-mm , and you say Hand held....
I'm impressed. It says a lot about the quality of every piece of the optical chain, and your skill a whipping that thing around.
Credit where credit is due.....
Nice. As I said earlier the pix look a bit low in contrast, is that due to the amount of atmosphere you shot through or just your choice in processing combined with the small JPEG?

I have to admit these appear sharper than I ever would have expected by stacking TC's.
Usually results suffer significantly.
Now a Nikon 400/2.8 will be an exceptionally sharp lens, and the fact that the 1.4 is built inn means it is perfectly matched to that lens, so it is not surprising that that combo will be sharp.
Adding an additional 2x seems to have worked fairly well. I can't tell about critical sharpness but it looks good.

One thing it proves is that starting with a great lens is really important, followed by optical matching of the converter. There does seem to be a reduction in contrast, but I don't know if that is optical or processing.
Interesting

In my previous answer I only mentioned Gamut obliquely, and @Nikonbrain above mentioned that Metal is often done on Dye Sublimation printers and could have permanence issues. Modern Dyes ARE better than older ones for fade resistance but Pigment inks are the best in that regard.
He is also correct about the relatively thin aluminum that is used, which might require some extra care.
Unmentioned is that most large print houses either do not use pigment inks or can do so if you specify it.
On your particular picture it looks like it does not need a particularly wide color Gamut, and might benefit from the pop of a glossy metallic print. But that may not be true of your next image. Consistency may or may not be important to you, so this is just FYI
If you are serious about quality have some test prints made, and you will know for sure.
The last issue is file submission, if they require JPEGs, or you choose to supply Jpegs you are going to get 8 bit color and the sRGB color space. Some processes are limited to that anyway, and many images fall completely within the sRGB color space so it is ok. But the best Pigment based ink jet printers (usually with 10 or 11 Inks) can easily exceed the sRGB Gamut, but you will need to supply a 16 bit TIFF to take advantage of that.(If your printer accepts TIFFs of unlimited size)
Good Luck, try it out with test prints

Nikon & Canon have been making really good cameras for a long time. Their higher end cameras are very well made and very rigid. But long telephoto lenses can be very heavy, and very long (increasing the lever effect)
The momentary shock of a bump or even small drop multiplies force greatly. The Nikon Tech was probably right.
Lots of people do carry long lenses with body attached, and fine companies like Think Tank make bags specifically for carrying the combination. My guess is that USUALLY the practice will be safe, but you have proven "Not Always"
That is really a shame. ....and a cautionary tale for the rest of us.
My Wife & Daughter shoot weddings, my wife shoots a Canon crop body with a 70-200 2.8 and a flash as her most used lens. She bought a roller bag where she inserts the camera with the lens AND Flash mounted into the bag. There is tons of unpack repack in wedding photography (Brides Home, Church, Location Portraits, reception etc)
I told he when she first bought it that flash is going to tear the shoe off the top of the camera, of course she did not listen to me, and in ten years her camera never broke, still going. So cameras are tough, but it only take one time.

......And, you were not even comparing noise reduction, because every different exposure & Iso had a different need for noise reduction.
There is no saving this effort, and certainly not by concluding everyone in the world is wrong except you.....

It may help you decide if you think about what you want most in a print.
There is no wrong answer because it is a matter of preference.
Metal prints are dead flat and often have a shiny reflective surface under the ink. This can make it look almost like a back-lit transparency. This increases detail and contrast.
Canvass prints buy design have a textured surface which scatters light and lowers contrast and detail.
Traditional paper runs the gamut from textured to silk to smooth and from Matte to glossy. There are even 'metallic' papers that mimic the metal print look
You will find photographers who love each kind.
Other considerations include the collective look of your photographs when displayed together-- different types rarely work well together.
I have seen my photographs on nearly every type, and while Metal prints do really Pop and show detail well, personally I always come back to pigment prints on a wide gamut paper like the now gone Ilford Gold Fiber silk Baryta paper. (there are others available) It best mimicked the classic double weight F surface dried on screens (rather than ferrotyped)
I do my own printing on an Epson 5000 run by the ColorByte Image print RIP (which replaces Epson's driver)
My best advice is to choose for yourself by having Bay Photo make one of each in a smaller size. Then you will know.
I have a friend who shoots ocean waves and has them printed and mounted behind 1/4" Acryllic in very large size. He does very well with them and they are beautiful.
Because of the graphic nature and great detail of your picture, it will look great on anything that enhances that detail.
Personally I prefer Fine paper, but will freely admit Metal or Acryllic are attention grabbers.

Try the links above first, but here is my experience with those issues....
I have occasionally seen similar issues on a Mac, but rarely.
The first thing I try is to restart the machine with the card and reader attached, whatever the initialization routines are, they sometimes help.
I have seen cables be the culprit, try changing cables.
Also check the card reader manufacturer for software updates.
It also sometimes happens that an OS update breaks things that used to work