That sounds like a you problem. I respect the terms of their license, and have no intent or desire to distribute any modified versions of their software, should I ever even care to try modifying anything in the first place.
If you do happen to find ways to improve their software, maybe consider applying with FUTO…
They’re not worried about individuals like ourselves making our own private modifications if we so care to. They know 95%+ of individuals out there aren’t even coders anyways, they just expect the few that do happen to go out of their way to modify their apps don’t distribute their mods without authorization.
The main infrastructure of their license is to make sure the big companies out there can’t legally rip off their code, alter it and sell it under their own branding or such.
They don’t give a flying crap if you modify the source and compile it privately for yourself, they just don’t want people or companies making profit from their works or distributing said modifications without their approval.
The app is so relatively new that initially it wasn’t even on any of the app stores, only straight from their site. But now it’s also on Google Play and APKPure, possibly other alternative app stores as well. Give it some time, I’m pretty sure F-Droid is on their radar as well sooner or later.
The app is still labeled as being Under Construction, but so far I’m rather pleased with it and would recommend giving it a try.