Authentik, Authelia, Keycloak, KaniDM come to mind.
That'd be identity providers / authentication servers or SSO solutions. But with most (/all?) of them, you'd have to program the payment logic yourself.
I think there are webshops, platforms to sell online courses and ERP or eCommerce software that can do both payment and authentication. I'm not a expert on that.
I think most solutions are either custom solutions that have been programmed by the people themselves (at least to some degree) or some of the big, commercial (and proprietary) platforms to sell online courses and memberships.
But don't search for "userbase [...]" that's a term I've never heard of. Search for "membership", "identity management", "single sign-on", "eCommerce" and "Stripe" (because it's one of the largest payment providers. And I'd have a look at the eCommerce world. Usually it's difficult to find something good. Most of them want a share of your revenue and aren't entirely open source. Maybe something to sell online courses with, is more likely to have the things you need.