Kanidm

It has a modest UI for end-users to handle self-service scenarios, and an app portal for OpenID Connect configured applications.

The backend is fully CLI based.

It is very robust and performant, built on Rust.

Yet very much in active development, so do not expect full parity with commercial alternatives at a feature level.

I run it to provision users for my home Linux devices and it supports offline login, my Homelab servers, and my self-hosted web applications through OIDC.

A nice ‘one stop shop’ for my purposes.

EDIT: Actually their comparison page might be a worthwhile read considering your question OP.