That's fair, there probably are more people with a suitable Android phone than with a FIDO or FIDO2 dongle, and you're correct that the phone (having more than enough storage) offers this feature and unlike a dongle I think you can be comfortable the phone won't "run out" of space if you sign up for frivolous nonsense this way.
try https://www.passwordless.dev/custom#heroFoot with the latest Firefox on a recent Android.
You can register and login with just a PIN code (or gesture pattern) from your Android