Indeed, just QML is insufficient for full compatibility. You have to redesign to make it cross-platform or use some cross-platform approach from the start (as modRana) or "translate" the app. Not sure whether translating that UBPorts app would be better than continuation of the available code. Have you tried to make some comparisons between them?

I agree with @r0kk3rz assessment - we are currently lacking people with skills taking the time for development. And its not clear how that could be resolved.

As for Matrix clients, lots of them are discontinued. Not sure why, but it looks to be all over the place when I checked at matrix.org. The client maintained developed by @r0kk3rz isn't the simplest piece of software, but I haven't looked deep due to commitments in navigation apps and will probably not be able to do so in a while.

But if you are interested in development of the client, even investing 0.5h per day could make a huge difference.