The gameplay is simple yet addictive, I can see it's potential.
Right now it's repetitive but it can be solved easily with different ennemy patterns (slow, fast, bouncing and coming back, coming in a spiral path etc.), and maybe some variation of the player "character" behaviours (moving, larger shield, smaller shield etc.).
About the art style, IMO pixel art and "flat" styles should not be mixed (UI vs "characters").
Also a "neon" style fits well this kind of arcade game, and can be very juicy. Maybe you should consider it if you if you haven't already done so. It's super easy to do, basically make the background dark and add bloom.