Some thoughts on the two cameras:

Nikon Z8 vs D850: It's FINALLY time for MIRRORLESS! T&C Northrup
https://www.youtube.com/watch?v=0KYiW8hhGqE

Z8 Vs D850 | Detailed First Look At How These Two Cameras Compare | Matt Irwin
https://www.youtube.com/watch?v=qGFOGGytMzE

Nikon Z8 vs Nikon D850 - Image Quality Review ZJ Michaels
https://www.youtube.com/watch?v=Ob02-bp_z5g

Nikon Z8 vs Nikon D850 Comparison: 15 Reasons to buy the Z8 and 5 Reasons to buy the D850
https://www.youtube.com/watch?v=jF9Y9C4F3pk

Cheers and best to you.

Yep, good glass is everything! Especially for landscapes and wildlife. I know you said you are mostly a landscape guy. You have great lenses for that. If you ever start thinking about wildlife, you will need more than 300mm. Don't let the naysayers fool you into buying BIG Bucks telephotos. I have been using the Nikon 200~500 f/5.6 lens for about three years now, and it's the best money I have ever spent on a big lens. There are better lenses, I suppose, certainly more expensive ones. But I have zero regrets and have not missed a shot yet because of inferior glass!

David, you know the UHH community is mostly populated with a bunch of retirees with more time and money than sense, right? Keeping up with this bunch of Joneses is a fool's errand. I had a look at Rockwell's Z8 review. Lots of great technology, and a lot of weaknesses vs the top two in the mirrorless camera market, Canon and Sony.

If you let the UHH community spend your money, nothing you have will ever be good enough to their standards. Digital cameras are plenty good enough now, especially anything released since the resolution standard reached 24MP across the industry, well below your D850. Buying another $4000 camera (or several), and even more new lenses, is well within the means of much of the UHH community. Only you can decide if that's the best idea for you.

My use profile is similar to yours. I decided to move from the D850 to the Z8 earlier this year, and I have been pleased that I did. Along with the move to the Z8, I switched to the Z line of lenses, and I do think I see a modest increase in IQ in most settings. From reports that I read, the Nikon F-mount lenses generally perform well on the Z8 with the FTZ adapter, but I preferred to switch over entirely to the Z system. The autofocus on the Z8 is every bit as good as the Z8 and probably better. I prefer the form factor on the Z8 over the D850 -- just a bit smaller and fits the hand better. Overall, the versatility and capabilities of the Z8 are impressive.

The D850 is a wonderful camera, and it served me well. That said, the Z8 is what finally pushed me into the mirrorless world, and I am very glad that I made the move. There are bean counters who will argue that the marginal improvement is not worth the cost (depending on your circumstances), but sometimes life is about more than just beans . . .

David, I shoot with Nikon cameras and glass. I have both the d850 and the z9. Most of my lenses are F mount Nikon lenses that I use on both cameras with good success. If you shoot carefully and properly, I doubt if you will see any difference in the images taken with a z8 and your d850. One difference that could mean a lot to you is that the z8 allows you to see exactly what the camera will capture. If you get the z8, you will end up loving it and will use it for most of your shooting. I got my z9 with one S lens, the 100-400mm. Then I got the s24-120mm f4.

You have the greatest 46MP DSLR Nikon ever created. Where is this GAS coming from?

There are many magical enabling technologies of mirrorless cameras. I've copied my own list of ideas, below. But, unless you have more money burning a hole in your pocket, you don't need to lust for another full-frame digital camera until your D850 fails from old age and long, long use, probably something that won't ever happen in your remaining lifetime.

Regarding image quality, remember that a digital sensor really doesn't know that a mirror has been removed from between the sensor and the lens. More importantly, ongoing investments in sensors and digital processors (the computer technology that converts the sensor data into an image file on the camera card), these investments now only occur in the mirrorless line of cameras. So, by definition, image quality only gets better on new mirrorless cameras, as those are the only cameras getting better.

But, taking the mirror away from one 46MP sensor to create another 46MP sensor and paying $4000 to get an EXPEED 7 processor over an EXPEED 5, that's not cost-effective, nor likely to generate any relevant differences in image quality, even with the addition of new (& pricey) mirrorless lenses.

Now, if that pep talk doesn't lead you away from a GAS attack, consider why I love mirrorless? Let me count the ways:

1. Smaller body by removing the space needed for that yesteryear flapping mirror.
2. IBIS that turns every lens into a stabilized lens, even the golden oldies of the manual focus, pre-electronic ages.
3. EVF with shimmering highlights of the plane of focus.
4. Histogram available for display in electronic view finder (EVF).
5. Detailed 10x zoom in EVF.
6. Silent (not a single noise) shutter.
7. Extreme frames per second, 20+.
8. Image playback in EVF for review when LCD impossible to see in bright light.
9. Image playback at 10x zoom in EVF.
10. Sharper mirrorless lens designs.
11. More ultra fast lenses as sharp wide open as any stepped down DSLR lens ever.
12. One digital platform supporting portfolio of legacy DSLR lenses, legacy SLR lenses and modern mirrorless lenses.
13. Autofocus beyond f/8 for extended configurations.
14. Platform for any / all ongoing technology advancements in digital photography.
15. EVF that displays the results of the image before shooting.
16. EVF that allows for ad hoc updates like boosting the ISO for a brighter focus view, then restore ISO and shoot, without even lowering the camera from a shooting position.
17. Entire frame available for selective AF position.

I'm sure I missed probably several others.