This is a very well conceived and executed idea with fun appealing art and sound. Super confident game with a lot of good going for it.
I found the combination of predictable wave based combat and upgrade pathing to be fairly engaging and self explanatory. The game performs well and the player momentum and weapons feel quite good. I like that you made it super obvious when you are recharging for the next shot to be available. Your gameplay music is really adorable and catchy.
I found the rotating spread to be confusing at first. For a couple minutes I thought the amount of time I held down my shooting button indicated the spread width, until I eventually realized it was rotating between 3 states. I think this could be a great upgrade, but I didn't catch on to how it worked, and I didn't really enjoy the rotation after I did realize how it works. Though there was some challenge in timing my shooting spread, I mostly treated it as a non spreading weapon in practice, if that makes sense?
I also miss the feature that is in Isaac where you can hold down the button to shoot. For accessibility reasons, I had to quit at your cool Boss level, because the repeated motion was a bit taxing on my hand.
Great job!