A lot of people suggest having an extra slot, but i think that it can be worked around by implementing more interesting ways of getting hearts. Maybe trinkets, giving a passive effect until your next shop? Or passives that give you hearts under some conditions? for exemple: every enemy a charger kills drops a hear/ some% for an enemy to drop a heart/ have your babies drop half their half cost when they die
This would also help with making the no heart-giving baby achievement actually achievable!
I was also thinking that Blood Puppy could be a passive that randomly takes place of a used charger after every shop.
Otherwise, great job on the game so far, can't wait to see more! Don't forget not to overload with work yourself either, dunno how you're doing with that, but quality over quantity remains way better anyway!