As far as focusing speed goes, when I used my D850 for wildlife and birds in flight, it was on par with my current Sony a9. And I am talking about eye focus, I used GROUP AUTO FOCUS on my D850 and my keep rate was at least 98%. I used the Nikon 70-200 2.8 FL lens when shooting on the street, and again, I hardly ever missed a shot.
Look, you have, in my opinion, the best DSLR ever produced.
If your happy with what you are getting, by all means keep it, if your not getting results, then it may not be the camera, and the Z7 or Z7II will not change what you are currently getting.
Below are some street parade grab shots with the D850 and 70-200.
Again, if your not getting what you want now, maybe it's time to look inside you instead of looking at a Z, and start concentrating on technique.

I purchased the d500 a 20.9 mp DX camera and was sometimes disappointed with the shots taken with the 18 to 300 that purchased years before when I had the d5000 a 12.5 mp dx camera. The lens was always OK with the small pixel camera, but sometimes soft with the higher pixel camera. I still use the D500 but not very often.

The d700 is a 12.1 mp camera and does not need great glass.

My guess is the lenses you have are not good enough for the d850 to shine. I had the 28 to 300 FX zoom and the camera produced shots that were under whelming. I have read articles by professionals that say the same. I returned the 28 to 300 lens and bought the 2.8 24 to 70 and the f2.8 70 to 200 lenses. Both are heavy and expensive and produce tack sharp shots all the time I take the time to set the shots up correctly.