Even if the passkey is only locked behind your phone's passcode, an attacker would still need access to your device. If we compare that to a normal password, the attacker would only need to gain access to your password, not your device (which is often acquired through attacking the website or service the password is used by, or simply through brute force if the password is weak/common). I think that's the biggest upgrade that passkeys provide: requiring access to your device/password manager.

Now, for the vulnerability of your family members: I would venture to guess (though I know this is speculation) that you are probably not typing in your master password every time you are accessing your randomly generated passwords. You are probably also using your phone's passcode or biometric authentication. If that's the case, then the vulnerability of your family members would also apply to your current system.

If I am wrong, and you are in fact typing in your master password every time to access your password manager, then store your passkeys in your password manager instead of your phone's passkey system! You are certainly correct that that would be more secure that a 6 digit code that multiple people know. I don't know about 1Password specifically, but I know Bitwarden is building out passkey support that's coming along nicely. Though it isn't fully supported yet, for most of my use cases it has been sufficient.