To build the GLUT test, you need to create a new CMakeLists.txt for it (see the VTK C++ examples). The CMakeLists that you used (the one External/Testing/Cxx) can’t be used for building the test as a stand-alone example.
I don’t think that building your own wheel will fix the issue with QML-VTK. Most likely, something has changed between Qt5 and Qt6 in the way that it sets up the OpenGL context.
As I said in my previous message, the best place to start is to find out where in the Python code the GLEW error is being triggered.