On paper I should love Authelia, I’m a sucker for y’all configured services, I can write a couple of files on my Ansible and boom, everything works… However I never had much luck setting Authelia up, Authentik on the other hand was very painless (albeit) manual (via UI) configuration. I don’t do anything crazy, so any of them would work for me though, I just failed on setting Authelia and tried Authentik and had had no reason to change.
I don’t think you’ll find a replacement because the distinguishing feature for CS is that it’s a service game that you can play online with other people, any game that is not a service game will not be the same, because the single player campaign will be finite and playing against bots gets boring very fast. But that’s okay, as far as service based gaming goes CS is not bad, it doesn’t require payment and at least last time I checked the micro transactions were all cosmetics so no pay to win either.