> And why do most of these “Auth0” replacements only implement nextjs?

Probably because it's hard to support different languages. Even if all the replacements support OIDC (not a given) there are still subtle differences in implementation and integration.

That said, check out FusionAuth! We have over 25 quickstarts covering a variety of languages and scenarios. (I'm employed by them.)

https://fusionauth.io/docs/quickstarts/

We're free if you don't need advanced features and run it yourself, as documented here: https://fusionauth.io/blog/fusionauth-on-fly-io