Userbase is a great open source solution: https://userbase.com/
This post has a bunch of other options that look viable: https://medium.com/visualdevspace/9-awesome-tools-you-can-use-to-build-membership-sites-e173e912c8a7
Headless CMSs like Ghost might offer something similar: https://ghost.org/vs/memberful/
The zoo of awesome backend frameworks that would perfectly suit your use case is huge.
Thus, what defines the choice is not so much the frameworks themselves but the availability of suitably trained/skilled people to you. See what skills you can get, and from there choose the tools.