OIDC, which Keycloak Supports, is a tad more versatile than IndieAuth. In my experience, IndieAuth is quite a complicated dance to perform compared to OAuth2, especially since in OIDC, JWT tokens are fairly established to enable apps to get user data.
KC's tokens can get you quite a lot of insight and data about a user if you want, or nothing if necessary. IndieAuth uses HTML scraping for that.
Aside: I can't count the no. of times I've referred to Aaron Parecki's OAuth2 Simplified page.
EDIT: I guess Keycloak's federation options are limited to LDAP or AD but can be extended with some effort?