I liked the "secret mechanic" as an implementation of the theme, but I don't think hiding it was sensible. I assumed it was going to relate to getting hit, so I tried that, and didn't notice any difference. If you're going to make the player work the mechanic out for themselves, I think it needs more feedback. Maybe ramp up the music, or show a combo counter, or change the enemy colour. Anything to let me know that something changed.
Do the enemies always spawn from the corners? Try using Quaternion.AngleAxis with a random value from 0 to 360 to get a bit more variety.
The shooting was well polished, it felt satisfying to shoot, and more satisfying to land a hit.