I love the arstyle.
And I like the idea for this game. It's simple but has enough depth to it that it makes for a bit of fun.
Maybe I just didn't read the instructions carefully enough or it wasn't mentioned in the game but at first I thought the shiel was buggy. It took me some time to notice it only works once the bullets turn yellow.
And there is one thing I would change: Don't have the enemies fire bullets. And here is why I think that would make the game better (keep in mind that this is just a suggestion and I'm basically spitballing here but maybe you could think about it):
I think the screen gets filled up with bullets enough just by the player shooting and missing.
Because you don't really know when an enemy will fire you are weary of getting too close to an enemy, so you shoot from a distance which means that you miss more often and the game gets pretty tough rather quickly. And the fact that the shield doesn't block freshly fired bullets means I can't even safely approach enemies with my shield up.
I think the game would still be challenging enough if the enemies didn't fire and the gameplay would be a bit more streamlined, making the game a bit more tactical.
Of yourse, with more and more enemies spawning it would get hectic anyways but start out a bit more in control.
Which might highten the feeling of chaos later on if the player is able to build up some amount of control which then spirals out of control instead of basically starting with chaos.
I definitely had fun playing your game, though. And (maybe even more so) thinking about it. :D