My Z9 back up is a Z6II.
When I saw how much I liked Z100-400 and features and had the Z800 on the way last spring I took a leap of faith and bailed out of the DSLR and F mount world while the bailing was good.
D6, D850 and 2 D500 and a bunch of f mount lenses from Nikon, Tamron and Sigma from wide angles to 600 f/4 E were sold. I got really lucky and hit the used market just before prices dropped a lot and that paid for my Z9 & Z6II, Z800, Z100-400, Z400 f/4.5, Z70-200, Z24-120, Z50, Z14-30, Z 1.4TC and Tamron Z mount 70-300.