Do you have an example? I am pretty sure that a FOSS license which requires companies to pay is impossible.

Open Source guarantees that anyone can give the software to a company for free:

“The license shall not restrict any party from selling or giving away the software as a component of an aggregate software distribution containing programs from several different sources. The license shall not require a royalty or other fee for such sale.”

And it guarantees that the company can then use it freely:

“The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business […]”

Quotes from the Open Source Definition.

None. Those things are incompatible with each other.

How would encryption even make sense here? Up to the server, everything is protected via TLS. And if you don’t trust the server provider, you can encrypt all you want, but they can just read out the RAM of the VPS or they could have backdoored the bare metal hardware to do the same. As long as the server has to somehow work with the data in question, the decryption keys have to be somewhere in there. And what do you mean by code integration? We’re talking FOSS here, how could someone prevent me from removing any “is everything encrypted?” checks in Mastodon? Also, what does the encryption on other federated instances even matter? Without having any in depth knowledge about Mastodon, your user agent will hardly be sent to other instances, and when and what you posted is meant to be visible.

What’s missing from the existing ones?

That’s what I’ve been using for a few years now, with SimpleCalendar (soon Fossify) on my phone. Didn’t have any problems yet.