Yes, unfortunately, due to the nature of the game, if you have maxed out a character, it will be hard to make newly introduced characters a long-lasting experience.
I was thinking that easy difficulty could increase (maybe double or even triple) the amount of love that you get from girls. and maybe increase the love cap to 150-200 (for all difficulties).
So that a new player that wants to focus on a single girl in easy difficulty will easily get somewhere that way. and that the maxing out of stats will at least be delayed as much as possible, since that brings to the situation that you described, where you can just breeze through any new character and story in a few minutes.
You could add unlockable "tokens" that increase the love of a girl as you reach particular milestones, instead of permanent stat boosts.
eg; "X character's item: increases love gain from X character by Y amount."
That would lower the grind with characters that you've already "completed", without affecting the progress of newly added ones.
You could even make those tokens toggleable in some sort of inventory system, so players can choose which grinds/girls to skip, and which ones to keep because they enjoy the progress/scenes or whatever.