I got a Z9 about 2 years ago and sold my D850 and D500. Initially I used the Z9 with a Z7II, as I was reluctant to buy another body as big as the Z9. When the Z8 came out, I bought one. It certainly pairs much better with the Z9 than my Z7II did, at least for wildlife work.
What makes sense for you may, of course, depend a lot on what you do and what you value most.
At this point, for a wildlife-oriented outing, I usually take the Z9 and Z8. If shooting birds, I'd favor the Z9 on the lens I expect to use the most, given bird ID AF. I don't use the vertical grip that much, but appreciate the larger capacity battery, the bird ID AF, and at times the built-in GPS. I think the Z9 also balances a bit better with my Z 800 mm PF than the Z8, although the Z8 is certainly usable. There are also times when I prefer the Z8 for its lighter weight -- say a longer hike -- or for its smaller size -- say in my kayak. For me, if there are drawbacks to the Z8 they are mostly (1) only 2 customizable buttons on the front of the camera versus 3 on the Z9 (forces some differences in customization between the two bodies), (2) the fact that it does not yet have bird ID and (3) to a lesser extent, a few other areas where the Z8 firmware is a bit different than the Z9's (understanding that some differences are required given physical difference between the cameras).
For a more landscape-oriented outing, I usually take the Z8 and Z7II. I tend to shoot the Z8 more than the Z7II, even for landscapes, and the Z8 also gives me good AF in case we come across wildlife. However, on a river trip through the Grand Canyon last year (before I got the Z8), I used the Z9 as my principal landscape camera, as we had limited charging available and I liked the extra capacity of the EN EL 18d batteries (and took several extras).
I still have a Z6II. The Zf looks better than that, but I would prefer something with more customizable buttons. Will be interesting to see what a Z6III might offer, although for me, at most it might replace the Z7II and Z6II. I'll certainly keep the Z9 and Z8.
Not sure if I would buy a body used; would depend on the seller and circumstances. But Nikon has had some attractive refurbished prices for the Z9 lately.
[corrected a typo]