I have considered maybe replacing the moon texture for the skybox and mobbing its size, but i feel like this would result in a low detail image, and im not certain i could get it to emit as much light as i would need.
I have also considered simply making a very large sphere, but it would be low in detail, there are limitations in size, and i feel like this might not be emissive enough either.
Billboard gui, depends how you apply but if the moon is going to be relatively small this is good option as you can adjust light emission. You can also make the “star” out of a sphere then use a billboard gui to give it that “glowing” effect.
Maybe instead try parenting the 3D object to the camera, and moving the moon some distance away from the camera’s position every frame (but not so far that your detail is lost). This will work okay except in situations where there is something that should be occluding the moon at a further distance than how far your custom moon actually is.