Would like to cheerlead for fully opensource Zitadel project here however.
Main repo https://github.com/zitadel/zitadel
Zitadel team clearly understand OpenID, Auth0, Keypass, etc specs and have all previous experience to implement identity management right for SaaS, B2C and B2B project scenarios.
SaaS Product with Authentication and Authorization https://zitadel.com/docs/guides/solution-scenarios/saas
Simplify Your SaaS: Multi-Tenancy and Delegated Access Management with ZITADEL Organizations https://www.youtube.com/watch?v=Cx_WgyY4TOo
ZITADEL Roadmap https://github.com/orgs/zitadel/projects/6/views/1
Zitadel took a very good direction into allowing to "build my own login and register ui"
Sprint Demo - ZITADEL 2.28.0 https://www.youtube.com/watch?v=hpQ4zrV48LY
[Epic] Login API and improvement of Register API #5015 https://github.com/zitadel/zitadel/issues/5015
https://github.com/zitadel/typescript
Previously had a look at Ory, Keycloak and many others.
Found those solutions either to be more "enterprisy" and over-engineered rather than something which can co-exist in my small team brain.