I respect RedHat and its great to see how they built a profitable business model around free software, while also being a substantial contributor. So it is a good example for sustainable free software business models. But I don’t think the RedHat business model is compatible with the F-Droid community in general. The RedHat business model is entirely focused on “enterprise”, meaning large companies, and F-Droid is pretty much the opposite. Requiring payment per downloads, per-instance, etc. fits in well in “enterprise”, it is quite incompatible with the much more individual nature of F-Droid users.
GitLab is perhaps a more relevant model: selling services to “enterprise” while allowing cost-free use of its “open-core” product and gitlab.com. I’m opposed to supporting “open-core”, what I think is relevant there is that only a small percentage of users actually pay anything, and that is enough to pay the bills. GitHub is built on a very similar model (though not free software): a tiny percentage of users actually pay anything.
Nextcloud and Owncloud are better examples, they are 100% free software. They require no payment to download or use their software. They make money by selling services and support to “enterprise”.
F-Droid of course is not really the same situation as any of those. It conceptually feels like an “app store” to most users, since iTunes and Play dominate when it comes to getting mobile apps on mobile devices. So there is a common assumption based on that experience that F-Droid is the place where a payment should happen. I think we can build upon that expectation to financially support free software developers while respecting the idea that access to software needs to be open and democratic.
One random idea would be to mark apps that require large payments. What large actually means is really a judgment call, and will be quite different in different contexts (countries, social class, etc). I suppose there could be a settings slider that lets the user set what level is a “large payment”.
I think there is also a common misconception on the side of developers who are charging for their apps: users who do not pay do not provide value to the developer. It is almost always true that even users who work to avoiding paying are providing some value to the app developer. That comes in many forms that are usually hard to quantify. Things like bug reports, beta testing, network effects, free promotion when users tell people to use an app, etc. Companies like Nextcloud and GitLab have figured out how to put that into a working business model. gitlab.com is free to use because those users are the free QA testing. Nextcloud spends very little on promotion since the users who install it themselves without paying then talk about it and what to use it in their jobs, etc.
All that said, I think that the payment model that best suits F-Droid, and the apps in it, is one where the user sees a prompt to pay the developer with a suggested amount, but the user has the option to specify any value, including zero.