RapidAPI Marketplace is the news buried in this article, at least for me. Sounds like a pretty neat front for your API that does all the payment / user management for a 20% cut
The down side is that you don't own any of the users. You don't even know who they are.
Is user management for an API / backend only service that difficult that it's worth giving over all your users to a 3rd party service which simply forwards on their requests?
Throw up a POST /user endpoint which returns an API key, and implement auth and... you're done.