@gassed
Lot of opinions in this thread... not sure how many have any experience with the cameras you're considering, but there are some decent points being put forward.
I also shot with the D90 for years, then moved to the D500, then the D850, then most recently to the Z-8. There were reasons for each of these "upgrades..." and bluntly, I still shoot the D850 over the Z-8 (in particular situations) but the Z-8 is both significantly faster in subject tracking and slightly easier to shoot (giving very similar results).
As an aside: for those claiming the D850 is a "better" camera, and that the Z8 can't take similar images, all I can say is... "get a horse..."
Try shooting with the Z8, it's easily a better camera for macro, and it's more flexible for w/a.
The one situation I continue to shoot my D850 is on blackwater dives (with the Nikon 60mm lens). This combination (for blackwater) is literally unbeatable - incredibly fast AF, etc. It is better than the Z8 and the 60mm (leveraging the FTZ adapter). I'm leaving for Anilao in a few weeks, and I'll take both rigs...
Having said this, the Z8 with the Z version of the Nikon 105 lens is signifcantly better than the older 105 on the D850 (AF speed and IQ). I continue to shoot with the older WACP on the Z8, as well as the newer WACP-C when I want something more compact.