As a dedicated Nikon shooter, I have to admit the "Z" line is going nowhere. Released in 2018, Nikon's lens releases are taking too long and no 3rd party lens manufacturers have announced compatible lenses. Contrast this to the Canon EOS-R, SIgma has just announced compatible "R" lenses are in the pipeline.
I'd go with the D850, you already have full-frame "F" glass, and the capabilities of the D850 are incredible.
Rumpelstiltskin has some catching up to do.
Nikon has a pretty aggressive roadmap for Z lenses.
Also, pretty much every F mount lens is compatible with the adapter.