Could you please elaborate on this solution? I'm not sufficiently knowledgeable about OpenID to quite understand what you mean, but I'd like to avoid any of the mentioned SSO providers, as they're all blocked on my systems for personal use.
Added: Found these as per mention in your post:
[0] https://openid.net/connect/
... so I assume you mean that I could install one of [0-2] along with Headscale [3] to get the similar effect of installing Tailscael, just without those annoying SSO providers? I will see if I can find the time for examining that solution. Anything that can keep MS and Goog away is most welcome
Keycloak is quite a complicated system to configure, though, there are easier alternatives out there. If you're just trying to get anything up and running, something simple like Authelia may be better for your use case (disclaimer: I've never tried it, but it seems light weight and other people online seem to recommend it).
I have no idea how the official clients will deal with that, though, but I've never used tailscale myself.