Qt's QML uses OpenGL and can at least render into an FBO in your existing OpenGL context more or less out of the box. Integrating it with a game render loop deeper should be possible too, but more effort.
Sure, for Qt 6, so at some not-yet known point in the next few years (I doubt they'll get that done next year). Whereas both the FBO method or a custom renderer are things you can do today.