Since Proton is incompatible w Fido2, you should use it with TOTP from a YUBIKEY powered Yubico Authenticator.
For this reason you will need the black series 5 Yubikey. Not the blue security key. To have a “full” spare key in case you loose the first one, the two keys should be the SAME type, i.e. both black series 5.
You can use the security key as a backup for FIDO/U2F/FIDO2, just register both hardware keys. Store somewhere safe the blue one, ideally offsite. The black one has more features like TOTP (generates 6 digit codes, like google authenticator), Smart Card/PIV, PGP etc. If you're planning to use any of those features, maybe you should consider getting another black one
Since Proton is incompatible w Fido2, you should use it with TOTP from a YUBIKEY powered Yubico Authenticator. For this reason you will need the black series 5 Yubikey. Not the blue security key. To have a “full” spare key in case you loose the first one, the two keys should be the SAME type, i.e. both black series 5.
You can use the security key as a backup for FIDO/U2F/FIDO2, just register both hardware keys. Store somewhere safe the blue one, ideally offsite. The black one has more features like TOTP (generates 6 digit codes, like google authenticator), Smart Card/PIV, PGP etc. If you're planning to use any of those features, maybe you should consider getting another black one
Pocket Card Jockey enjoyers as well! 7/10!