@bluxomat To answer your questions (and my understanding of your needs): If you have different customers on one Cloudron instance, there is no option to have a "kind of" multi-tenant user management. If you give the role "user admin" to one user of client A, she is able to see and create all users on that Cloudron instance.
In that case it's better for all (including Cloudron the company because of more subscriptions) to install a Cloudron instance per client. If you are a "frugal fox" (never heard of this before, I had a quick translate of the german word "Sparfuchs" that means something like: If I don't have to put more money in other people's pockets, why should I?), then the only chance is to have some kind of "multi-tenant" within the user management of the apps themselves. But if your customer has more than one app on your instance, you have to repeat the creation of users in every app again and again.