Put your entire UI and the image with the game “viewport” inside an Overlay widget (a widget that stacks it’s children on top of each other), then control the size of that widget.
To prevent the “true” camera from showing anything, well, just point it away from your world. It’s 2D, after all, if the camera is looking the other way, it will display black. The SceneCapture2D becomes your “real” camera.