Adding more characters doesn't change the optimal way to maximize Mana growth, but . It is optimal to impregnate the most mana-efficient character and assign everyone else to Emily. The discount makes them even more mana-efficient, there's no incentive to switch to interact with other maidens until you can Hyper the discounted character.
That's not true, I'm wrong. The story, meeting other characters, seeing their various images, advancing their storyline, those are all incentives, only they are not (currently) improving your mana growth. This could change if there are characters that give mana more efficiently locked behind a quest. In Albero Verde the double impregnation sessions gave twice as much mana for the cost, but some of them required a quest first. There could also be one-time mana rewards for some quests, that's an incentive that can work.
But those incentives rely on you finding out about them, in my case I jumped straight at the obvious way to maximize mana, which is making Emily work overtime and holding a Festival every day. With those hidden mechanics I would get penalized for ignoring the story, which is entirely fair.
Actually, another way to incentivize interacting with more characters would be to make the Festival take a full day, wasting that day's mana, not even sending it to Ollie. This would make it optimal to spend roughly 25 days before the discounts add up to the mana you would lose by holding the first Festival. This gets faster on subsequent Festivals (as discount take up a bigger part of the remaining mana cost), then once characters reach max discount the optimal duration gets longer again.