Good job, I liked basically everything about the game, I would just like to see little more enemy variaty (maybe dashing enemy, "shotgun" enemy etc.) and maybe (but thats just speculation) the ideda would work slightly better in reverse, where you would add the time to the timer and on one hand you would try to power up as much as you can, but on the other the enemy scaling would be getting agresive enough that it would force you to leave certain upgrades behind.
Overall I have had fun with upgrades and the game as a whole. Good job. :)