Maybe passkeys to a relay window that stores your keys in an encrypted OPFS (Origin private file system) vault with a mix of a pin and FIDO2 HMAC secret ("hmac-secret" extension)

https://docs.yubico.com/yesdk/users-manual/application-fido2/hmac-secret.html

https://fidoalliance.org/specs/fido-v2.1-ps-20210615/fido-client-to-authenticator-protocol-v2.1-ps-20210615.html#sctn-hmac-secret-extension

https://levischuck.com/blog/2023-02-prf-webauthn

https://developer.mozilla.org/en-US/docs/Web/API/File_System_API/Origin_private_file_system

I have an old Solid demo with a relay window login

https://ldux-meme-scratchboard.netlify.app/