You can use any passkey provider app. I work at Bitwarden and we’re building mobile passkeys for android right now. We can do the e2e sync, but if you want you can always self host Bitwarden server and just use our clients app.
The BitWarden passkey dialog irks me because it makes me click the passkey I want, even if I have exactly one. It would be better to have a feature where I could specify "always use this passkey and don't prompt", since that's what I need 99% of the time.
This has been annoying me as well: WebAuthN even provides metadata that lets authenticators know which credentials they're willing to accept, so at least in that case (usually the flows where you have to enter a username), auto-selection should be possible.
With discoverable credentials (which Passkeys by definition are), i.e. the flows where you don't even enter a username and the website learns it from the selected passkey, I don't think there's a way around a key selection process, but the UI can definitely be improved to distinguish the two.
Maybe something like "website XYZ is trying to verify your account 'username' – is that ok?" vs. "website XYZ wants to authenticate you – which passkey do you want to present to them (if any)"?
I'm already seeing Bitwarden as an option for Passkey authentication on iOS! Apparently the app already exposes itself to iOS as a WebAuthN backend (or the API is the same as that used for password managers).
Unfortunately that API doesn't seem to be wired to anything in the app yet, so selecting it inevitably fails.