> By design, you cannot create a backup of passkeys on a hardware key

In fact, I think it's technically possible. But it's true, as of today I don't think anybody supports it.

I was experimenting on solokeys with ios, in principle we could backup Passwords (the ios/mac app) into a solo key, space permitting, including passkeys, regular passwords and totp (not wifi passwords). I believe the same is for Android, but haven't tested yet. This is all experiments I've been doing on my own, there's nothing ready to be released.