I think that having a secondary mechanic (maybe something that requires you to stop, but deals more damage) would be nice. Game starts to become repetitive after a few upgrades.
As for the upgrades, I think that you should focus on adding changes to gameplay instead of just changing stats. For example:
- Add push back force
- Add option for bullets to pierce through certain number of enemies
- Dash ability
- Weapons (different shooting pattern)
basically, something that will require players to change their strategy and adapt to the game.
Pretty simple and polished game though. Well done!