The gun customization is really cool! It was really fun trying out how different mods work together with different mods.
Some minor things I'd change is:
-Make the tutorial text write faster, I was waiting for it to load instead of reading it
-Maybe have the game pause when you get a power up so you can read it, and decide where to put it. It was a little difficult to keep track of what did what while fighting enemies at the same time.
The biggest improvement you could easily implement in my opinion is making way more enemies spawn, and make them way weaker to keep the balance as it is. As a person who loves this style of game, I've found that blowing up 15 enemies that all have 1 hp and do 1 damage, is significantly more satisfying than killing 1 enemy that has 15 hp and does 15 damage.
Anyways I enjoyed playing your game, love playing twin stick shooters in jams and you definitely a better and more creative take on the them than my own!