I picked up my Z9 late Tuesday - finally!
In key respects, there are major differences compared to the D5, in others it perpetuates much in the jizz, ergonomics of a Nikon pro ILC, the menus particularly. I have toned down the fps to 15fps for CH, 10 for CL (likely to tamp down to 7fps), reserving 20fps for Recall Shooting Functions setup. RSF is assigned to Fn3, which is easy to access shooting the camera in horizontal or portrait formats. Single-Point+AFOn to Fn2; WArea-Small+AFOn to Fn1, WArea-Large+AFOn to Sub-selector button, and AFOn so far is on 3D Mode for BBAF.
Despite the chagrin about 'failures' of subject-detect/tracking of the Z9 reported in many forum posts, no flagship MILC is a point&shoot (!), and nothing much changes. It pays to be primed to toggle/swop - and fast - between AF modes on a challenging target....e.g. bird or mammal moving within clutter. MF with Peaking works superbly when needed, even on a fast telephoto.
After reading all the debates about the RAW formats I've gone ahead and set image quality to High Efficiency*. I've no plans to use 15 nor 20 fps in machinegun mode, but have ordered a pair of Delkin cards from BandH, but this will take a few weeks across the pond. I did check the 64gb Lexar CFEXpr Pro after a few shorts bursts, and it was distinctly warm. Anything more sustained may cause overheating....
I've tested my Z9 so far only with the 500 PF, including with TC2 III, with which the focus is still very fast, and 3D easily locked on to a flying Yellow-billed kite, and at some distance. Don't plan to use this combo much, but compared to D850 and D5, the AF is impressive at f11 in responsiveness, and across much of the frame.
Contrary to what I've read in blogs and forum posts, it has turned out to be quite quick to change Shooting Banks, Metering, and Silent-shooting via the EVF with the Z9. The loss of the Metering button from camera top-left is not as serious as I'd feared. In fact it's looking better already to have fps there in its place. Changing AF settings is also much faster - Assign Red-Record to scroll the Front-Rear dials. Overall, Nikon engineers are to be commended for facilitating a Custom setup to control almost all the key controls with the right hand
and not leaving the EVF. This now includes Playback, which is one big challenge to muscle memory after so many years on DSLRs - i.e. when chimping to keep trying to press top left of the camera!
All this is an experimental setup...so early days, but the setup file is archived as backup to this computer incase of a vital roll back to 'Last Known Good'. Finally, the other snippet of advice I applied is to do a full Reset all Settings' of the camera before setting it up. Some fairly experienced Nikonians believe this clears the memory. Nothing to lose with a new camera, or setup file has been backed up as restore is very quick off a card.