Regarding 2. that is false, I build everything on a CentOS 7 docker and my appimage just works on every distro from that era onwards (~2015). Hy do you say that it's pretty nasty ? It's the same packaging mechanism than on Mac and Windows, the app ships everything except libc.

There's a linuxdeployqt tool for appimaged - I've never used it but saw a fair amount of apps using it I guess it works ?