I've just started Book 2, and I've just gotten a hunch about Sayumi, and I hope I'm wrong, or else I'm going to be very sad :(. I already had a clue from the menu screen of her, and this just solidified it. I'll come back to this once I've finished up to the current update.

Hunch:
I think Sayumi trying to get the MC a harem will lead her to not be with him anymore. I feel the reason the MC married her was because everyone else left, or something along those lines. But something happens to the MC in that future, perhaps he regrets everything that happened with the other girls. Sayumi keeps reiterating that she's the one who loves him the most, so I have assume that whatever future she's working toward won't have her in it. She loves him that much she's willing to do that. Once the harem is set, I assume that would change the future and something will happen to her. At least, that would be the expected result. Judging from these type of stories, maybe they would do something to save her, but who knows. She also from the future, I can't expect she could stay forever. Perhaps she eventually disappears because of altering the past, but as a result, the MC meets her younger self, and they restart their relationship.