We could not figure out how to achieve this without pure QtQuick3D without major modifications.. In the end, for this we opted to go with Qt3D instead of QtQuick3D. it seems that Qt3D is more geared towards customizable rendering while QtQuick3D just covers the standard cases and little else.

I noticed that .obj files does not contain material information in them, instead blender creates a .mtl file along with it. As far as I know balsam does not accept .mtl files along with .obj so instead I tried other formats. I tried all formats in https://doc.qt.io/qt-5/qtquick3d-tool-balsam.html#supported-3d-asset-types . GLTF2 (.gltf, .glb) format worked for me. However the other formats that blender can output did not give material information.

That's very promising indeed!
Thank you very much!

have a nice day!

Cheers
Manfred