This game sadly suffers from dubious gameplay experience. Starting from level 6 I had no trouble on any ordinary level but would get stuck on boss levels. That means I would seep through 9 or 10 levels in about 3 minutes and then get trapped in repeat loop every 30 seconds on boss level and have to grind for almost an hour getting upgrades to be able to counter long range, rapid firing, high splash enemies.
Technically you can reset your builds and change profile of your 'army' but it just doesn't feel comfortable without presets. Also, the rounds last only a minute, two tops, so I feel discouraged from tinkering with my units, especially since at later stages I can hardly even tell what I could optimize.
I'm not going to give you an advice on how to fix your game because I don't even know what you're aiming for, but I can tell you that the gameplay is just not satisfying. I really like the aesthetic, though. And the game loop is interesting - at least on paper.