Thank you for the feedback, it means a lot to me!
I didn't want the player to be able to buy out the entire upgrade shop right away — I wanted them to progress gradually. I agree with you about the penalty for mistakes — it's too harsh, I didn't have time to balance that properly. Yeah, by level 10 there are too many missiles, and I think that again comes down to the balance not being fine-tuned. But I made the decision to keep each game session short, which is why the constraints are pretty brutal, like the high number of missiles. I believe that within the scope of the game, the final upgrade levels for each stat give an optimal boost to the player's power — anything beyond that would be overkill. But maybe it felt different to you because of what you described in your first point.