Worth noting that implementing FIDO2/Passkeys is more challenging than it looks both from a UX standpoint and from a threat modeling standpoint. We tried to cover some of this in a blog post, in case anybody is interested: https://www.slashid.dev/blog/passkeys-security-implementatio...
Co-founder of SlashID here (one of the companies mentioned above). I think we have exactly what you are looking for:
https://www.slashid.dev/blog/anonymous-users/
https://developer.slashid.dev/docs/access/concepts/anonymous...
Hit me up if you want to chat more: [email protected]