The issue there being there's a big usability headache with enrolling multiple devices. You really want one device to be able to enroll all your devices (including not-present and offline), but there's no mechanism to do this with the way the webauthn spec works at the moment.

None of the password managers (including but not limited to ones built-in iOS/Android) work that way. The Apple one (and I think Google is the same) keeps the private key inside the secure enclave (security processor), but it is still copied to each new device - though it is end-to-end encrypted during that transmission.

That’s how I use them. Passkeys on two Yubikeys. And I tag in my password manager which credentials have what form of auth. UP, TOTP (also stored on the two Yubikeys), Webauthn or passkeys (the former indicating 2FA).