I don't need a backup camera, ultimately. I'm not a pro. That said I have the Nikon D6 as back up to my Nikon Z9. Which I've turned to twice when the Z9 was in the shop. However. Enter the 600 f/4 TC lens which becomes a paperweight if the Z9 fails (e.g. I can't shoot new lens on old body). Further, enter my upcoming trip to Baffin Island in the early spring where I really don't want to be without a backup camera (e.g. be there and fate delivers a failure of the Z9 -- afterall it'll be over a week of very cold temps without respite for the gear, or me, from the cold). So the thought of spending several thousand dollars for ANY backup camera for just 10 days (and any future such trips, of course) hurts. Option is to risk it -- one camera body -- but that's really not a rational option, especially given my luck
Option 2 is to take the D6 as backup AND take up a backup lens (say the 500 pf with tc or even the other behmoth lens 180-400)) but that's really pushing already strained gear packing space.
Option 3 is to choose the Zf for the backup camera. Bonus here is I suspect I'd likely wind up using that camera quite a bit for street and walk about photography; even portrait work I do (despite my preference for 'big' cameras in hand). So that would give it greater use than 'emergency' use only and take the sting out of the (already less $ than the Z8) price tag. And it's got all the AF power of the 8 and 9 so would be a great option for wildlife if it comes to that (eg Z9 crash).
Anyway, thought I'd use this forum to put my thinking down on the 'page' as I move to a decision. And with this crowd I suspect there will be a few thoughts and opinions
Also, on another matter re the trip north. I've decided for a Zoom option (along with the 600 prime) I'm going to take the z 70-200 and a tc, likely the 2x)