I really like the game itself. It's look very polished and made in short amount of time.
There is a lot you can do there with that. And at some point it's a bullet hell when there are a lot of enemies and they die in a delay time. But it adds to difficulty, so at some point you don't want to kill them one by one.
But i'll swap the appear animation with death animation, because for me, it's more obvious when somehting is about to explode and more fitting. And when the enemy die, you could just make it bland and maybe animate.
But anyway, as for 6 hours, that's a lot :)