Yeah, that's basically what I meant. After a while, I felt like I was repeating the same actions without much increase in difficulty or any new changes to the gameplay, so it started to feel a bit repetitive. Some gradual increase in enemy count, difficulty, or new mechanics could help keep the progression noticeable
It's also possible that I was playing in an unexpected way. I was drawing large squares that eliminated most of a wave in one go, just by moving my mouse from one corner to another while camera is far away from the ground. That might have made the game easier and reduced the sense of progression