@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.
I'm just thinking digitally here, but most fonts, Wordpress themes and plugins (and other paid themes and plugins), etc. are for one installation/project.
As you mention it, first fonts are not in the same category as Wordpress which carries a General Public License (GPL) which carries the obligation to ALL themes and plugins to be produce and distributed under the exact same condition, meaning they must be GPL as well.
Limiting the number of instance installation in the case of themes or plugins for Wordpress, or any FOSS which comes with such license of freedom, is among the biggest SCAM going on in this sphere right now, as this is a violation of the GPL license which, among other things, stipulate that once you have a copy, you can freely copy and distribute it under the same license's terms. Hence, technically speaking, if a wp theme developer does limits you in the number of instances you can install, then he's breaking the license himself which they still pretend to be GPL which is an absolute non-sense.
Clourdon is NOT GPL, nor even open source if I'm not mistaken, right now, even though most of the software part of the offer are. That's not a problem at all. simply because you do not absolutely need a Cloudron box to run any of the software offer which are coming with GPL. When one pays for either cPanel or Plesk the principle is the same, the prices are always PER INSTANCE, obviously, and that is even though they too offer GPL apps installation via their dashboard via Softaculous and the like.