Yeah, I should add a special hit sound for the boss and probably some form of vfx, like sparks when it gets hit. Pretty sure that was actually on my to-do list at some point, but it got lost in the rush.
It's interesting to hear your thoughts about the Overcharge ability. I think you may be underestimating its actual impact, as the game's overall difficulty is quite sensitive to player damage output due to the fact that all enemy spawns are time-based. Adding even one extra use of Overcharge would actually be a fairly large balance change, especially since it does quite a lot of damage when used against the boss (about half of its health!). I'll probably revisit some things I thought of but never got to try out in order to make Overcharge feel more appropriately game-changing, like adding even more bullets in a wider arc (which would miss the boss anyway), or having it wipe out all enemy bullets when used.
In any case, I'm glad you liked the game. Thanks for playing!