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/
Is this a problem that state and country remain exposed ?
Personally I would recommend that you make up your own domain privacy scheme.
A P.O. box + virtual number should do. Right ? As long as the contact is reachable you would be within Icann rules as far as gTLDs are concerned.
It's correct that the amount of whois data revealed may depend on a number of ...