Thanks for mentioning ZITADEL.
Co-founder here. Supertokens is a good solution with obviously a lot of open source traction, which is great to see in this space. Expanding to authorization makes a lot of sense. ZITADEL supports both authentication and authorization in a turnkey solution (AuthN, AuthZ, APIs, UI, DB). Looking at Supertoken's roadmap, Zitadel seems to be more feature rich offering Passkeys, OTP, multi-tenancy, account linking and a management ui.
Zitadel looks interesting, but their website is quite confusing. I'm not sure is it open source and free to use (as keycloak) or you need a commercial license to use "open source".
The software is open source under Apache 2.0 (https://github.com/zitadel/zitadel). No open core or similar, we run the same version on our Cloud Service and for Enterprises. Thanks for the feedback, we need to make that more obvious then.