Well, if you are upgrading just for weight savings, I don't think going from a D850 to a Z8 is going to make a huge difference. The Z8 is about 100 grams lighter than the D850. However, some of the Z lenses are lighter than their F-mount counterparts, so if you start acquiring some Z-mount glass, you will start to notice a bigger weight savings.
What kind of photography do you like to do? The Z8 (and the Z9) are tremendous for wildlife photography and video, among others. If you are a landscape shooter, one of the biggest benefits will be the EVF and it's ablility to give you a WYSIWYG preview of your shot. There are other technology improvements that will help out a landscape shooter as well.
The D850 is a very good DSLR camera. I had a D850 as well, and it was my favorite camera for a number of years. But ultimately I relized that the future of camera technology was going in the mirrorless direction, and it was going to open up a whole lot of improvements in focusing modes, subject detection and tracking, better optics and image acuity and much more. So I sold my D850 and eventually all of my F-mount glass (except the 500 PF) and moved over entirely to mirrorless.
So I would suggest you look closely at the reasons that you want to move to mirrorless from your D850. If it is primarily for weight savings, then the Z6iii, or Z7ii (among others), should give you the weight savings you are looking for and get you into the mirrorless arena,
If you are a wildlife shooter, I would urge you to strongly consider the Nikon Z8. It is very similar to the D850 in ergonomics and form factor, but will be a huge improvement in autofocus modes and subject tracking capabilities, among others.