Wouldn’t it be better to use envelope encryption and sync the passkeys? I mean with envelope encryption somebody would probably be able to create a rfc that would support cross browser/password manager sync a passkey by using a encrypted envelope that would be able to be deceptively wither via tpm or by using credentials so when syncing it would ask for upn/password of the original device or something so it can even be e2e encrypted, so some kind of federation protocol for password managers that you own