This was a lot of fun! It looks like I have played your updated version. I found the difficulty to be very forgiving, because I didn't die until I hit the boss, and I lasted a lot longer than I thought I would. That's a plus for me in this style of game. I really enjoyed the power up mechanic and being able to choose which one I got. I also appreciated the visual cue aspect with the enemy coloring to indicate difficulty and that the player is progressing.
I do think the information boxes and text need to be larger. I thought there was no information about the power ups until I realized it had been there, it was just too small. I also think that the menu pop up for the power ups could be optimized further, with larger text, maybe a UI design that gives the information in fewer words, like boxes with images that have a short phrase for each. Maybe even reducing the number of choices to three would help simplify the display of information for those? I would start with making it bigger, though. That might be enough.
Lastly, I would love to hear some music and sounds. Different sounds for the power ups, maybe a sound to signal when new level enemies show up, and especially boss music would all be nice polishing touches for this.
Great work!