I absolutely loved the combat mechanic.
Building momentum and spinning up the sun is really satisfying, although it's a bit slow when you're also surrounded by bullets. I managed to get the moon to bounce horizontally, which was also satisfying, but perhaps the ball should lose momentum eventually. The moon was a lot more difficult to use, because you have to rebuild momentum every time you pick it back up. Also I often pick the moon back up before I want to, or right after I launch it if I didn't build speed. It would be nice if picking up the moon was another button press.
If the mechanic was more fleshed out and a bit less finnicky I would see myself playing a full game with it.