First of, thanks on the pointers for QCad, did an attempt some time ago, but failed trying with Qt6 (didn’t know about the possibility to do this with Qt5 - granted didn’t hunt it down).
Things I left out was patching for libexecinfo and the “loop” you mentioned, one file I had to patch for missing Qt5Script headers, recipe is updated here, if you still want to have a go at it at haikuports I could guide you through the process, otherwise I’ll probably update the recipe myself. 
EDIT: first I’ll try to get a build for Qt6 (checking up on Arch recipe, it should be possible)