Flatpaks are generally a last resort solution (the binaries are often non-native), but this is a desktop application where bulletproof reliability is not as critical. Consider using that instead. I may be recommending the Flatpak to all those older platforms in the future. We'll see. The listed problems are significant hurdles.
It is unclear whether I will be solving these problems. There does exist a Flatpak however:
https://flathub.org/apps/details/im.riot.Riot
Flatpaks are generally a last resort solution (the binaries are often non-native), but this is a desktop application where bulletproof reliability is not as critical. Consider using that instead. I may be recommending the Flatpak to all those older platforms in the future. We'll see. The listed problems are significant hurdles.