Would be great if you added support for security keys such as YubiKey
Here's a PHP library you could use https://github.com/lbuchs/WebAuthn
here you can see my version of it. I'm just storing two columns returned from the library, credentialId (base64 encoded) and credentialPublicKey.
Also, if you add support for YubiKey, you can apply to be added to the YubiKey services catalog which may help increase the number of people who know about Sellix https://www.yubico.com/works-with-yubikey/catalog/?sort=popular