This is quite fun, and I enjoyed playing it a lot. My only complaint would be that after you reach a certain point, you're basically invincible. After around 15 minutes, I actually got up to eat lunch, came back, and had an insanely high score. Still a really fun game though, I just think the difficulty could max out a bit higher.
Yes, I struggled with getting the difficulty to be maintained over the course of the game... I've added an update which will increase the difficulty some by changing the mechanics. But this is still definitely something I'll work on in the future. Also am planning to make the endgame more interesting by adding more mechanics that begin to come into play then.