I suggest you break yourself of DSLR habits. You need to embrace what you can do with a mirrorless camera and set up and use it as such. Trying to apply your D850 or D6 ways is going to handicap you.Decided to finally part with what I still consider a very solid DSLR, the Nikon D850. Had an opportunity to trade on a nearly new Z9 (ever-so-slightly used). Have a fair amount of good F-mount glass to start with so will be getting the FTZ-II adapter to take full advantage of that. This new body will complement a Nikon D6 which will stay in operation (may never give that one up).
So was curious if I could get any helpful, good to know tips from others that have moved from the D850 to the Z9. Realize that I'll be tasked with familiarizing myself with a new system and slightly different array of controls (always part of the adventure) but would welcome any insights. Hoping my familiarity with the D6 may help make me feel at home with the Z9 but that may be wishful thinking.
I also predict if you take the above advise your D6 days are numbered.