I won this game on second try :) Very good game, fun, kept me entertained and interested all the way through. I actually passed over a "boss room" and it scared me so I continued to farm for upgrade. Was some tense moments. And I eventually reached another boss room that I completed in two passages. I liked the retro controls and the pleasure of upgrades.
Maybe another upgrade to augment turning speed would be interesting. Also, in retro games like Galaga or Roketz, too much of something (speed for example) is sometimes bad depending on the player and its control style. Allowing player to orient their style between agile, bruteforce, etc, could be fun also.
Actually if the difficulty kept rising we could reach insane level of upgrades :)