Postgres does have ldap based auth, and also can authenticate against AD using sspi/gssapi.

The problem with that is that it requires users to have been created inside postgres first, and that you can't manage group membership inside AD.

Or Kerberos authentication (which AD also supports) https://www.postgresql.org/docs/current/gssapi-auth.html

Yes, and that AD LDAP interface is riddled with MS specific deviations that require complying with MS’s way of doing LDAP.