If there's an auth spectrum, from:
roll your own -> use a language specific library -> use a full featured solution like Auth0, Firebase or FusionAuth
it seems like SuperTokens is in between the library and the full featured solution.
I guess I'd ask, if I don't want a full featured solution (the use case you've identified), why wouldn't I use a language specific library (devise for rails, passport for js, etc). Or is the value that you're providing a modular integration so I don't have to integrate authentication + authorization + user management + API auth myself?
Anyways, congrats on launching. We need more auth solutions and I'm glad to see an interesting approach in the open source world. The other major contender, KeyCloak, is, I've heard, a bear to run and extend.
Full disclosure, I'm an employee of FusionAuth.