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’d say yes for home use that’s perfectly fine.
Lots of people here teach you the 3-2-1 rule. Which is how it’s supposed to be and stick to that if you’re a business or have valuable data… But that’s also not the whole picture.
I think more important than the actual number of backups is to make sure they work. I’ve seen computers where the backup or cloud sync failed and no one noticed. And after the harddisk got damaged they got aware of the fact that the last successful backup ran 9 months ago… Or people started to save things in a different directory and that drive wasn’t part of the backup. Or the backup was encrypted and the key got lost together with the original data.
I personally am a bit cheap on the third backup. I replace that with an old external drive and copy my vacation pictures there every half a year or so. Just don’t store that next to the computer so everything burns down together. I’d say that’s more than enough. And your cloud backup already does 99% of the job. It’s at a (physically) different location and does all the really important tasks (for home use.)