In D8040#150577, @davidedmundson wrote

If you do want to set this based on the initial scene backend, skip using this method and call the static method on QQuickWindow instead.

After reading through the docs I think the combination of both is fine: We can use the QtQuickSettings to have a base. But if we init a QPainter compositor we just enforce software through the QQuickWindow API. That should (TM) work as well. Dynamic switching is then not required. AFAIK the only "dangerous" combination would be QPainter compositor and OpenGL compositing. The other 3 combinations should just work fine.