We (Saasform - https://saasform.dev/ https://saasform.dev/) are building exactly something like that. Since payments are heavily tied with authentication (and s

We (Saasform - https://saasform.dev/ https://saasform.dev/) are building exactly something like that. Since payments are heavily tied with authentication (and since authentication and payments rarely are part of the core business) our idea is to delegate these two key (yet non-core) features to an external service. We are currently building our MVP and we plan to release the first part as OSS by the end of the month. Eventually the whole core will be open source and there will a version as a service (a-la GitLab). The first release will be focused on startups to help them to launch faster, in the next versions we will add more growth/enterprise features. Feel free to drop a mail to dave <at> saasform <dot> dev if you have questions, want to contribute or if you want to be among the first users.