It's a cool idea but I didn't get very far x_x. I think the big bullet would feel more powerful if after its first upgrade it could go through the first enemy before despawning, after the 2nd upgrade go through 2 enemies before despawning etc. Also I had difficulties maneuvering the vehicle with the keyboard, but I feel like controllers would feel more natural. And for a better user experience I suggest to find a way to put the UI next to the player to avoid having it dispatched everywhere. For example a health bar above the player (with the heart icon), and slightly above another bar (with the shroom icon) split in n sub-bars to show the progress for the bullet upgrade.