Yes, there are probably a few attacks that need to be rebalanced :D
Have you played the game several times? The number of HP of the bosses depends on the order in which they appear. If the face appears first, it won't have much HP; if it appears last, it will have a lot. I realize that this is a rather artificial way of increasing the difficulty. Fortunately, it's not the only method I use :)
As for the hourglass and the transition between vulnerable and invulnerable, I'm actually thinking of adding a few sound and perhaps visual signals (although this doesn't mean you don't have to look at the hourglass for the more cautious ^^).
Anyway, thank you very much for your comment and feedback :D