I now feel stupid - I didn't know that you could press E to tether and I beat the entire game without it by estimating angles of reflection and adjusting accordingly. Just went back and tried it with the tether mechanic and it was a far better experience!
Only major piece of feedback I have is about the tether ability's range. It looks like the range that you can use the tether is sometimes greater than the glow of the planet, and sometimes it isn't. This wasn't too big of a deal, but it did throw me off sometimes because I would shoot my ball at what I thought was the edge of the planet's range, only to not be able to activate the tether.