Using 1Password as your passkey backend seems like it would solve all the problems you're describing, except that passkeys are (for the time being, at least) locked in to 1Password [1]. It works on multiple OSes (as a native passkey backend on iOS, via browser extensions providing WebAuthN on all major OSes) and isn't tied to your Apple ID.

If you care a lot about exportable passkeys, Bitwarden (and Vaultwarden) can export them. Not sure if any other implementation can import them at the moment, but the data looks portable enough (i.e. it contains the ECDSA private key and all other client properties required).

[1] https://support.1password.com/save-use-passkeys/ mentions that "Passkeys saved in 1Password can’t be exported at this time."