I created diffs that can be applied to version 5:
https://github.com/davrot/ldap-overleaf-sl

However, I couldn't / will not test them because I am too stupid to get an Authentik server running and was not even able to run a test OAuth2 in Python against it. Thus I will give up on the Authentik server idea and will modify the overleaf password part to be connected directly to a real LDAP server.

Long story short, maybe the diffs and scripts, I did last night, are helpful for somebody...