Fine tuning a f4 or f2.8 for a specific distance is fine. If you read what I was responding to first before responding you would see that the OP was taking about a f6.3 lens used at long distance. Which, as any competent photographer knows, fine tuning is unnecessary.
And I am glad you fine tune your lenses, BUT IT IS ONLY GOOD FOR THE DISTANCE YOU FINE TUNE IT FOR. (and this comes directly from Nikon, not me)

Because of it's better engineered lenses, and the fact that they are made start to finish by one company (not parted out to the lowest bidder) Nikon lenses DO NOT NEED fine tuned. I have shot Nikon glass for over 50 years and every lens, manual and auto focus have not needed fine tuning to get sharp images from every distance. My 20X30 competition prints speak for them selves. Not a rotten apple in the bunch.
I am happy that you like your Tamron, I am sure after you spend TIME fine tuning your lens it may give the same result as a out of the box Nikon.

The Nikon 200-500 is sharp throughout it's range. It is extremely sharp at 500. I have used that lens for almost three years and commonly show print at 500 mm to 20X30 without issue.
Again, the Nikon 200-500 is extremely sharp at 500mm.

Your quote, "The focusing AF and zoom features are restored back to factory specs and cleaned" is very misleading.
The vast majority, 98% of them at least get only a cursory check, to make sure they are working properly, most all refurbished lenses are actually lenses that customers have returned because they FELT they were not working properly. Most of these lenses have NO PROBLEM, they are run through a general check, they are not restored mainly because they do not have to be restored. And they are cleaned only on the outside, there is no INTERNAL cleaning. They look brand new because they are, they are brand new lenses returned by consumers, and since they are returned, they cannot be sold as new.
So, yes, refurbished lenses are a great value, the vast majority are not SUPER checked over, BECAUSE THEY DON'T HAVE TO BE, because their was nothing wrong with MOST of them in the first place.
I like to refer to these refurbished lenses as someone else's BAD COPY.
Finally, just let me say that the majority of the refurbished lenses are returned by very, very fussy photographers that would not be pleased if they could walk on water.

Sony HX99V, 24-720 mm Zeiss lens, pop up viewfinder, HDR at 6 stops. Carry in a belt pouch and you will thank me when you return from your trip.

You are referring to the current sale on Nikonusa.com that has the refurbished Nikon 200-500 on sale for $999.99.
https://www.nikonusa.com/en/nikon-store/refurbished-cameras.page
Many here have been very happy with Nikon refurbished products. Keep in mind that they also have a new Nikon 200-500 on sale for $1256.95 with a 5 YEAR WARRANTY vs. 30 DAY WARRANTY for a refurbished 200-500.
https://www.bhphotovideo.com/c/search?Ntt=nikon%20200-500&N=0&InitialSearch=yes&sts=ma&Top+Nav-Search=
So, this is the question, is $256.96 worth an extra 59 months of warranty?
Only you can answer that question.
As for the lens, it is my go to lens for wildlife photography, it is sharp, sharp, sharp throughout it's zoom range. YOUR DECISION. But either the new or refurbished will serve you well.

Here is the staple for Florida wildlife photographers who carry way more equipment than they need. BUT IT WORKS.
https://www.amazon.com/Baby-Trend-Debut-Stroller-Cascade/dp/B078227XTG/ref=sr_1_fkmr1_1?keywords=three+wheeled+baby+stroller+with+pneumatic+wheels&qid=1558784080&s=gateway&sr=8-1-fkmr1

I do not recommend a monopod for macro work. You are much better off with a tripod, one with an independent center post. Here is one that is on ebay for a good price, the photo's do not show it, but the center column can go horizontal, if you put a ball head on it, you will have a great system for macro.
https://www.ebay.com/itm/Manfrotto-055XPROB-Tripod/133061832376?epid=97346115&hash=item1efb1a6eb8:g:HXoAAOSwIJRc5xGu

Monopods and wildlife do not mix well, for me, birds in flight are too unpredictable for a monopod. You cannot follow fast enough. If you must, use a tripod with a GIMBAL head.

My wife and I have taken several river trips. As a result of this EXPERIENCE I now only take a pocket Sony camera in a belt pouch. Taking any DSLR is TOO much. My favorite travel Sony's are the HX90V and HX99V, both have a Zeiss 24-750 zoom lens capable of 20X30 sharp enlargements. And they have pop up viewfinders that act just like your DSLR.
The belt pouch keeps you hands free for those shore excursions.
Please do not take your DSLR, you will not be needing it if you take a pocket camera.

A tele converter will not work on your 18-300.
There has not been a tele converter made that IMPROVES image quality, in fact, the opposite happens.

1. I am so sorry for you that you are STUCK with Nikon, I know many feel differently.
2. I have found BENRO tripods reasonably priced and substantially built. They also come with a tool bag and carrying case.
3. INDURO is another great brand. The Enduro GIT403/404 are examples.
4. Benro TMA 38CL ser3M3 is another
5. TMA48CXL series4 mach 3 I believe is $437.00

Good question grass hopper, here is your answer.
https://www.youtube.com/watch?v=7XG5YcG3sxM

I am 70, nothing scares me anymore.

I own the Nikon 17-55. It acts like and looks like the Nikon 24-70. First of all they are both made in Japan, I stand corrected, the 17-55 WAS made in Japan.
I find this lens just as sharp as my 24-70 F/2.8, which is saying something because the 24-70 is SHARP. The 17-55 of course gives the same field of view as the 24-70 on your D500, which, I find anyway, an ideal lens for almost everything.
The 17-55 is not a light weight, I would not want it around my neck all day, unless of course, I used in on my favorite BLACK RAPID strap, then carrying that lens around all day would not be too bad.
The range of the 17-55 is better than the 18-35 especially at the top end, the 24-70 is almost a portrait lens, but also wide enough for good architectural shooting. The Sigma will give you a field of view of 27-52.5 mm. So it looks like the Nikon gives you more at both ends. The Sigma however is a 1.8 lens vs. a 2.8 lens, so chalk one up for Sigma, but with the ISO range of the D500, one stop is nothing for that camera.
The 17-55 is built like a tank, and will give you years of service and after years of service you should get almost what you paid for it on resale.
Good luck and keep on shooting until the end.
Yes, the 17-55 is a real value, I got mine on Ebay, Mint in the box for a really great price.
Good luck and keep on shooting until the end.

I shoot the 50mm 1.8 lens off the D850 without a loss in MP unless I crop of course.