Yeah, I hear you. By the 3rd level things liven up a lot. I had to tone down the first two levels, people were complaining it was too hard. Originally, the waves were much closer together.
When you hit the 3rd boss, you know you've made it :)
As a side note, I notice you had sandboxed the game. If you allow it internet access, It'll pickup the proper fonts, instead of the default Times New Roman. It uses Google Fonts. But it's only cosmetic, and doesn't change the functionality of the game :)