I was in a similar position. Going from a D850 / D500 combo to the Z9. I was initially using my D850 (which I love) as a backup, but then I accumulated enough Z glass that it wasn't going to really work as a backup anymore.
I hemmed and hawed about it (and posted a thread here too, lol), before finally settling on something. It really came down to either a used Z7 or Z6ii.
I ended up grabbing a used Z6ii for now. And, I'll be honest, I don't like it. I really dislike the control layout. I dislike the User Modes instead of being able to swap setting banks. I dislike the serious drop in AF performance from the Z9. I dislike that it takes a different remote connector than every other Nikon camera I own. I DO like the small and lightweight form factor, though. And the image quality is good.
It's going to be my backup for my big trips this year (Hawai'i and Costa Rica), but I don't anticipate using it much. Definitely not for birds in flight, anyway. It's really just going to be a breakglass in case I absolutely demolish the Z9 on those trips (unlikely, but it's a 'just in case').
I don't think I'd be any happier with the Z7 or Z7ii, either. My complaints are pretty much the same across both ranges. But I went with the Z6ii for it's lower cost and good low light performance. I figured that would best complement the Z9 for now.
And I know it's only going to be a temporary fix. I'm really just waiting for the mythical Z8 to finally launch, then (if it ends up being what I hope it is) I'll probably get on the wait list for one of those.
Someone on this very forum told me that, really, the only true backup to a Z9 is another Z9. And I gotta say, I think they're right. If the rumored Z8 doesn't meet my requirements, I'll probably look at getting a second Z9 in a couple years. Maybe a used one, when they start hitting the secondary market eventually.