I tried it out with the auto-rhythming on which certainly helped me a lot, thank you for adding such a feature :D
I think you might be right about the Panthrill, I felt that any time he grabbed me with for his dominant position, I would lose a heart guaranteed and when you can randomly get two+ of him in a single wave, that can be run ending. I don't know if it's possible but he could maybe appear in wave 8 onwards, if you want to keep the bonus round a challenge to unlock, but I would recommend limiting the amount of him that can spawn to be one per wave.
I had my stats up to 10 for both vaginal and anal and seven on oral and breasts and I was close on time as I think I found and lost my last heart to the final enemy of the wave. Maybe having a counter for how many enemies are left in the wave could be a nice touch as well, just so the player knows how many more enemies they have to encounter.

Regardless, I'm happy to see you're so receptive to feedback, I'm loving this game so much already and knowing you're doing your best to make it better is amazing! <3
I certainly love all the animations in the game, certainly helps to inspire me to work on my own pixel art games and animations too! :D