Originally I was hoping to have multiple levels to play through and maybe upgrades between, but it all got scoped out in favor of a more basic difficulty curve that shortens enemy spawn times. I agree with your feedback, it's either hard because you're not used to the mechanics yet, or too easy once you've gotten enough to just tear through everything. I'll try to address it soon!
High scores is a good idea too, especially if there are leaderboards to compete on >:D Thanks for playing!