I’ve become a Nikon person in the year or so.
These days I am not a really heavy duty photographer but I have a D3500 and P1000. The Z9 is one I might get.
There is not a thing wrong with the 3500 and there are some truly great lenses for it for pretty low prices. If you shoot in low light one of the more expensive cameras might be on your list. The Z9 does not take better pictures than the D850 but is has more features, one that I really like is the pre image shots it takes which is uncanny for sports.
I took some pictures for a friend of mine who is a a football coach at a local high school. Every picture I sent him seemed perfectly timed with the ball just out of the fingers or the foot having just touched the ball. There is no magic about it or skill for that matter you just set the camera to take 20, 60 or 120 pictures a second and for up to a few seconds before you press the shutter release. It is capturing constantly prior to your hitting the shutter button but when you snap the picture, that his hit the shutter it keeps the last 1, 2 or 3 seconds of exposures prior to your hitting the shutter button, it is really a fantastic feature.
You don’t have to purchase the Z9 for that though, the Z8 will do it also. The Z8 is about 2 thousand less expensive. The biggest benefit of the 9 over the 8 is the size of the battery. When I got the 9 there was no 8.
One problem with the 8 or the 9 however is that they are full frame cameras. Full frame lenses are more expensive. With an adapter (about $100) you can use the older “F” style as opposed to the new “S” style lenses. The newer are more expensive but some say they are a little better.
Good Luck.