The fun factor is the best part of this game and it can be improved further. Maybe try using a resource for the shield and ammo so it's not so overpowered and adds a layer of strategy in the gameplay.
I also found a bug where your score will be continued even after the player dies.
Also with the current score generation, it's easy to be abused. Example, one can just shoot away the asteroids from hitting the shields and stay playing on an easy difficulty/wave, like wave 1.