Hello Jack,
I am not sure how useful my opinion will be to you since:
- I've never tried a Z7, nor any of the S primes, and also none of the other FF mirrorless cameras (from other manufacturers)
- I am not a pro, so the way I use my cameras most likely does not match your (and the others') ways
I did not upgrade from the D810 to the D850 because I do not need the extra resolution, nor the extra requirements on glass and shooting technique that the increased resolution brings with it.
I decided to keep the D810 (and the Df too), so I went with a Z6 to complement it. The Z6 has less resolution (but a weak-ish AA filter), a little faster AF-C (especially in low light), and better video (though I have not yet got to it...) than the Z7.
In my opinion, Nikon's play with the Z system is not as much about the cameras (though they try to release competitive products) as it is about the new mount and the S lenses for it.
We are still in the early days of the Z mount, but I can already tell the the 24-70mm f/2.8 S is worth it. Can't wait to try the 70-200mm f/2.8 S (January 2020 based on the latest rumors).
People may like it or not where Nikon is going with the Z mount lenses, but I think there is no turning back. I believe that during the latest 'group retirement' wave, many of the old style lens designers left Nikon. I do not think that the classic lens 'character' will show up in their lenses going forward. The more recent F mount lenses show some of these changes (for example the 105mm f/1.4), and the best S lenses take this further based on the characteristics of the Z mount. It appears that Nikon tries to balance resolution/sharpness with something I call 'realism' for the lack of a better word (others call it 'clinical' - I do not agree with this when it comes to the 24-70mm f/2.8 S). I love the rendering of the 24-70mm f/2.8 S. The S primes may be larger/heavier than their F relatives, but are all better and all stabilized on the Z6/7.
So my plan was to sample de Z system while still having the D810 for more resolution, 3D tracking and faster action.
However, after a year of using the Z6 and a month with the Z50 I find my D810 and Df gathering dust on the shelf. I will soon force myself to take them out for a photowalk just so they don't feel neglected...
But all this happens as I don't shoot fast action, nor do street/candid photography. I also don't look for too long through the EVF at any given time (I take a few photos, walk a little, take another few) so there is no significant strain on my eyes.
So my suggestion would be to keep the D850 for a little while more, and to get a Z6 with the 24-70mm f/2.8 S (if possible), or the f/4 if you want less weight, and see how that works. Then, if you like it, you can replace the D850 with a Z7 mII down the road...
Even if the Zs feel like Nikon cameras, there are some differences in usability due to their mirrorless nature. It is now second nature to me to gently touch the shutter button or the AF-ON button as I raise the Z6 to my eye - when I get to look through the EVF, it is already on and ready to shoot