Thanks for the feedback and glad you enjoyed the game, about the difficulty it was hard to balance because of the upgrades system but we probably could have made the hard mode more difficult, if I return on the game some day, I will try to fix that !
yeah, unfortunately difficulty ramping with player upgrades is not an easy design problem! i think you could tune it by placing yourself in later levels with lots of upgrades and playtesting it... if there were a lot more enemies and bullets, the spheres=health mechanic would have felt a lot more meaningful