I have had a Z7 Since December of 2018. I liked it so much I bought a Z6 to go with it last year.
Like you, I am a long-time Nikon user. I can’t compare Nikon mirrorless to Sony or Canon which I have not used personally, but believe I may still have some relevant thoughts.
First, when you pick up a Z7 or Z6 (and I expect the same would be true of the Z50 and Z5, although I have not used one of those bodies), it feels like a Nikon and many elements seemed familiar. Of course there are important differences in the Z bodies as compared to the DSLRs bodies, especially in autofocus. If you get a Z body, you should get the mirrorless edition of Steve’s book on Nikon autofocus. Very good and very useful. And written from the perspective of a wildlife phtographer. Thom Hogan’s Z7/Z6 book is also very good. I have both.
Nikon F mount glass works perfectly well on a Z body with the FTZ adapter, as long as it has a focus motor — generally AF-S and AF-P lenses. Scew-drive lenses will not autofocus on the FTZ adapter, although they will work in manual focus.
While the autofocus system of the Nikon Z bodies is not as good as the current generation of Nikon DSLRs for fast action, it is very good and in many ways better in my experience for other situations. For stationary or slowly moving objects, I find I generally get better autofocus results with my Z bodies than with my DSLRs (a D500 and D850, which I kept for several reasons, including birds in flight and fast action). That may be because the autofocus points fill the frame, so you can always put on autofocus point where you want it. Or perhaps because focus tuning is generally not needed. Or perhaps because it will focus with f8 lenses (e.g., a 500 mm PF + 1.4x TC) and all autofocus points still work. And the Z bodies are decent for birds in flight. Yes, the D500 and D850 are better for birds in flight and of course the D5 and D6 are even better (although I have not used them). But the D500 and D850 are also better for birds in flight than the D810 that you have. I am not sure how the Z bodies would compare to a D810 — while I had a D810 in the past, I traded it in when I got my D850 and so never had it at the same time as the Z7 or Z6.
The Z bodies also work quite well for manual focus, with focus peaking visible in the EVF and with magnification modes.
Z mount lenses are very good. I’m not a pro or a lens tester, but they seem better to me than their F mount counterparts in many cases, espcially in the corners and at the edges.
The Z bodies lack the ability to change autofocus areas with a single button press and the EVF has a disorienting slideshow effect at the highest frame rates. These would be the first two things I would change if I could and hope they will be different in any Z7s/Z6s version released or other new model.
Since I got the Z7, I have used it more than any of my other bodies. I have used my Z7 and Z6 extensively for wildlife photography, particularly with the 300 mm and 500 mm PF lenses (with and without the 1.4x TCIII) and with the 70-300 AF-P FX lens. They are light and nicely balanced. I particularly like using them in canoes and kayaks and when hiking.
Of course, If I did not have a Z7 or Z6 now, I might wait to see what is introduced next. Hopefully that will be soon.