Yeah, I've beaten all the levels myself - at this point I can generally do 20 in a few attempts, but I spent a lot of hours just playing the game during development so I certainly wouldn't expect that from a first-time player. I think I made level 20 on day 3 and it took me several minutes to beat, which feels like a good length. My mistake was that as I improved over the rest of that day and the next day, I started finding the level too easy, so I decided to make it harder, not considering that it was already at a good difficulty for a beginner player.
Thanks for the comment!