I understand why some characters’ core values could make a one-sided harem feel difficult, but I don’t think it’s entirely incompatible with the narrative.
For example, you said Charlotte becomes “depraved ” if you corrupt her, but she could still remain loyal to the dick she’s obsessed with. She might only want to try the most intense, kinky things with him—such as public sex—similar to what another commenter suggested.
As for Iris, I think it’s fine that her early scenes include lesbian sex or group sex since she is not open to the idea of being exclusive but once there’s development with the mc, it wouldn’t be unbelievable for her to at least try. Or, in the case of corruption, she could become so obsessed with him that she feels women and other men aren’t enough anymore.
So I think the two parallel routes are the way to go. Besides, the game is about relationships, as the descriptions say, if the mc has the power to influence the girls, it’s natural that some would try to use it for their advantage to have that fantasy, even if is a bit unfair for certain characters.
I hope my comment was useful.