Nice idea to include that kind of progression system in a horde survival!
The game is really fun to play, with a great difficulty curve (I guess that highly depends on the order of flavors).
Everything is polished to a very good level. There is one funny bug that only appears if you defeat all the bosses, start the game over and click many times ¦3
Visuals, music and sounds are pleasant, the controls feel great (I would appreciate having an in-game cursor, as my own cursor sometimes was hiding somewhere on white background), and the game is almost always nicely readable (except in late game, where enemy bullets are very hard to see in the overall chaos).