I sold my D7500 (baby brother of D500) as well as the terrific 200-500 and all my F lenses to get the Z6 and z glass. I am super happy.
Currently , imho, the D850 IS the best DSLR in the full frame market to date. I wish I had one as it’s the camera that in ten years I’d say was the best in the world for at least three years. There is simply no other camera that I can think of that in a few years I might say that of.
if you have the means, go with the D850 , and get the z6ii or wait for the z8/9
Nikon z system currently don't have native wildlife lens and the autofocus performance reviews were not overly positive compared to either d500/d850, I needed something with the reach and within a certain budget. All things considered Sony fit my bill better and Steve's positive comments about the same helped.
D850 is definitely one of the best cameras but I wanted to switch to mirrorless.