It turned out that no (mainstream) passkey provider allows backups however, making them infinitely worse than just using passwords.
Maybe this will help, but fuck me, it’s all complicated, especially for a damn foundational security mechanism!
It could be so simple, just look at SSH keys, which I think largely use the same principle.
I have recovery mechanisms for regaining access if I lose all my keys, but (while I'll admit that the tooling for managing public keys in the general case is lacking) you're not supposed to (need to) copy private keys between devices.
Technically you can generate a GPG key and load that onto multiple Yubikeys, but that's not as secure as letting the Yubikey itself generate the key. Plus if you only use one GPG key and one of the Yubikeys is lost (but not destroyed) then you have to rewrite all the remaining Yubikeys with a new GPG key since you can't selectively revoke them.