Post-process effects like bloom would be a nice touch that elevates the graphics. I would change the graphics of the ships to feed into the bloom by having light borders instead of dark ones. Think tron. I would have bright particles for explosions, along with a shockwave effect when ships detonate -- with a nice juicy sound. You might consider screen shake for when the enemies explode near you; may or may not improve the juice of the game. The UI font is terrible, hard to read nor does it do anything to set the atmosphere of the game. It looks far too comical.
Shield mechanic should make the enemies flee, I literally had it on for too long and drove them off of the screen. Had to spend a lot of time dodging projectiles for them to finally appear back on the screen. This just wasn't fun. It is also too powerful the way it is, in that if just tanks all those hits and can be used to destroy enemy ships when they come to close. The shield needs a cooldown or some kind of limit where the gets depleted and then needs to be recharged over time, maybe via collectibles -- energy that is picked up from ships that have been destroyed.
The AI needs to be improved, in that all it does is charge at you when you are shooting and then flee once a shield is on. Lastly, shooting doesn't feel good, it lacks juice. You need better sound effects and probably particle effects to make it clear how impactful they are. I also think that the player shoots are too slow; maybe these get upgraded to being faster, but I was never presented with the option as those appear randomly. My feeling is that their base speed is too slow, they should be quicker than enemy bullets with the enemy ones slower so as to be easier to dodge.