The definition of a backup camera includes a couple of perspectives. I think of a backup camera as a camera to be used as a primary camera when the primary camera fails. In that role the ZF is a very good backup - it's a smaller and less expensive camera that includes many of teh features, technology, and hardware found in the Z8. In a similar manner, the Z8 or Z9 could be backup cameras with similar capabities. It's okay to have a backup camera that is different and below the capabilities of your primary camera. A Z7ii could be a good backup camera for a Z8, but it comes with an earlier AF system. The Z6ii is a good backup - but it is lower resolution and the older AF system like the Z7ii. Even a much older Z camera like the Z6, Z7, or Z50 can serve as a backup in a pinch. In this role the ZF is a very good backup camera because it carries a lot of current functionality and fills some additional needs such as a small travel camera or events camera. It can also cover the role of a camera for use with older lenses for a retro feel.
The other version of a backup camera is really as a second camera for use concurrently or replacing your primary camera. Some people like using two cameras at a time, and in that role you normally would prefer similarity. A Z8 is a lot like a Z9, but you could get a second Z9 or a second Z8. In this role the ZF is quite different in handling and controls, so it may not be a great camera for concurrent use in spite of the AF system similarities. As dual cameras it's a lot easier when both cameras have similarities. I also wonder if the feel in your hand with a ZF would be acceptable. But the other version of this is the value of a second camera that is different for dual camera situations. I might use a Z7ii or a Z6ii as a second camera if I already owned one. I might use a Z6ii or Z6 as a second camera if I wanted video capabilities and low light capabilities without much emphasis on AF. I might use a ZF if I previously had a DF and liked using a camera with my older lenses.
Part of the decision involves how much you want to spend on a camera that is rarely or never going to be a primary camera. Is budget an issue? Would you rent a second Z9 or a Z8 as a backup camera rather than owning a camera you rarely use. One point of view is using a great camera like a Z8 behind your Z9 has a very high cost per image because the actual volume with the Z8 would be low. If budget matters, you might use a prior generation camera as your backup because it minimizes the cost of depreciation.
I would only choose the ZF as a backup if I really wanted a ZF for what it is - not for the AF system for use with wildlife. That's a good secondary benefit, but probably not the main benefit of choosing a ZF over a future Z6iii or Z7iii. I would sit tight for now unless a ZF is what you really need.
