wow!! really fine gameplay loop you've got here :)
it was quite tough at first, till i figured out how busted it is to stack upgrades. each upgrade has a downside that sounds like a dealbreaker, but i've noticed they tend to naturally compensate for each others' downsides. even the scattershot sounds like a joke, but it's actually quite useful for increasing range. i made a beast of a build and managed to get all the way to wave 13!
i did notice though that there wasn't much point to increasing health. every time i died, it was because a group of enemies ganged up on me and i got overwhelmed. perhaps the health upgrade could instead create a barrier around you to help prevent this? one last thing, itd be nice if i could advance text with the mouse button, and scroll menus with WASD.
either way, love this game, and i'm excited to see what you do next! :D