Nice game, I enjoyed it even though it felt a bit slow for me personally.
I would have liked to see more interesting upgrades and overall the pace of the game feels slow. I was hoping that the lvl 2 ship would allow me to equip multiple upgrades.
Another improvement I suggest is to add more randomness to the movement of the enemy ships.
The difficulty of the game is ok since each bullet is very punishing and the game seems quite polished.
Good job!