Agree about the noise difference. I was shooting with my Z7II + FTZ + TC + 500 mm PF at a local heron rookery recently and a guy near me (shooting some kind of Canon DSLR), asked me what camera I was shooting because he was impressed with how quiet it was. And I was using the mechanical shutter. I also found this summer, shooting from my kayak, that I seemed to be able to photograph waterbirds with the combination closer than in the past. I suspect the lower noise helped.
At the moment, I have kept my D500 and D850. I think they are better for BIF and fast action than my Z7II and Z6II, unless you need to use a TC. (I have the 500 mm PF, so even the 1.4x TCIII puts you at f8, while the 1.7x TCII puts you at f9.5 and the 2x TCIII puts you at f11.). The ZIIs are better than the ZIs for this and reasonably good for the slower, larger birds that I mostly photograph (eagles, swans, herons, egrets, geese, cranes and the like).
Keeping the D500 and D850 is not a perfect solution to the BIF and action issue. I’m not sure what the percentage of my shots are that involve BIF and fast action, but it is not always predictable. So it would be nice to have one camera that did both very well. And for me, I’d like more megapixels than the D6, as I often need to do some cropping.
And I find that the ZIIs are better than the D500 and D850 for me in enough ways that simply using the DSLRs all the time is not ideal. For me, I find having focus points covering the full frame, seeing my exposure in the EVF, more accurate focusing on stationary and slowly moving subjects, and working better with TCs, make me choose my ZIIs more than my DSLRs at this point.
I’m hoping that the Z9 is a significant improvement. And that it will actually be available to those of us who are not NPS in a reasonable period of time.