This is obviously kicking the can down the road, but I "solve" this problem by storing passkeys in a third-party credential manager that supports them. That way I can use them on any device that I've installed the client app or browser extension on. I have this working on Fedora, macOS, Windows, and iOS.

But again, kicking the can down the road.

Well, you can until they use the attestation feature to block your passkey manager implementation.