Although parenting to the camera seems like it should be a good solution for a UI, it allows objects to "penetrate" the UI which will look weird. Optimally you can use 2 Q3D Viewports, and create an extra scene with a fixed camera and your UI objects with a 0 alpha background, and layer that above a view-port for the main games scene. If you really need to parent objects to the camera just make an dummy object which sets angle/positon to the cameras x/y/z position and rotation, but again for a UI this isn't the best option.
If you use a Q3D Viewport, You can make the view-port small to make it render a smaller area if the UI is only something like a bar at the top, it doesn't have to encompass the entire area. For 2D ui's it's best to just use the construct 2 canvas as a "layer" itself with behind mode set on Q3D Master.
Using extra scenes is a bit weird but the main things you have to know is:
1. By default, everything is created in the scene named "Default", and this scene is picked.
2. To use another scene, you need to first create a scene with a unique name like "MyScene", similar to how multiple cameras work.
3. To add objects / change properties / do anything really with a specific scene, you need to pick it with the "Pick scene" action in Q3D Master, similar to how multiple cameras work.
4. Once the scene is picked, any actions that have anything to do with a scene will affect the "picked" scene. e.g. change background, parent an object to a scene, create an object, change fog, etc. the only exception are cameras which exist independently of any scene (they're global to all scenes, so they don't exist in any one scene).
5. To move objects between scenes you need to use the action that parents them to the scene in the hierarchy section. so you need to pick the scene you want to move them to first.
6. To affect the default scene again, just re-pick it with the "pick scene" action, the name it uses is "Default".
7. To rendering multiple scenes is as easy as writing the scene name / camera name in the viewport properties and layering them as you please. Semi-transparency won't render properly between viewports though due to renderer design.