Hi Stueh,
Another self-professed non-expert here.
Just gave this a whirl on my RPi3B+ and it compiles and runs OK. Runs like a complete dog though. I regularly use the “community build” on my even less powerful RPi3 which handles full HD and multichannel audio.
I think the poor performance might be down to a lack of video acceleration in the QT5 - some interesting reading here for anyone else interested and the lack of multichannel would appear to be more a default kernel compilation thing rather than config. Two big mini-projects for me anyway.
I had hopes of running native PMP on top of Raspbian as the PMP community build process appears to have stalled at 2.55 (it’s always been a bit hit and miss when it gets upgraded).
Anyway, thanks for an interesting few hours looking at cross compilation from Ubuntu, compiling QT5 etc. Might have to get a RPi4 now 
