I thought you executed well on the idea! I was going to suggest that the game needed power-ups, but then I played a second run and realized I just hadn't gotten any the first time round.
The power-ups are really nice. I love the way they stack, and it balances the game really well in two ways: You are you incentivized to move towards the enemies with the slow moving bullets so they're actually a threat, and as more enemies come in per wave, you also have more opportunity for power-ups which allows you to keep up.
I would suggest trying to dynamically adjust the randomness so that if you go a long time without getting any power-up drops, the probability of getting one increases, and vice-versa for when you do get them, so that you're less likely to get long streaks or droughts.