I purchased a mint used Z7 and returned it in one day. the build quality and lack of features present on the D8XX series disappointed me. while the live view was nice it will not replace my D850.
my main reason for the purchase was macro using live view. also, I would never use the Z7 for wildlife, etc.
I was very disappointed with the Z6 when I bought it.
It didn't feel right, and viewfinder was confusing/disappointing.
It took me about a month using side by side with a DF to come to love it.
Now I can't go back...
Maybe you can try again when Z8 comes out.
D850 is still a very good camera. If it does what you want, then no need to change.