Authelia is popular, as is Keycloak. I believe Red Hat develops Keycloak or at least has a hand in it.
I'm on this journey as well, figuring out what I'm going to use. Currently most of my services just use LDAP back to AD but I'm looking to do something more modern like SAML, oAuth or OpenID Connect so that I can simplify the number of MFA tokens I have.
Just as an anecdote you may find useful - Personally I used to run an Active Directory for Windows and FreeIPA for my Linux machines and have managed to simplify this to just AD. Linux machines can be joined, you can still use sudo and all the other good stuff while only having one source of truth for identity.