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.
I think most people who win the lottery for example, make bad choices and end up losing that money anyways.
I’d say it’s possible to a degree. But it’s certainly not easy. If you own a bigger house, you need someone to clean it. Same for other nice things like a big pool. A large, well-maintained lawn etc. There will be people envying your wealth and you got to deal with them. You need someone to manage your money and assets, or have lots of spare time to deal with all of that. And it’ll be at the cost of your hobbies. If you have too may people doing stuff for you, you need people to manage those people…