As of this point, Mitsuki should end things with Kashiwai. From the beginning, there was always something strange about how she wasn't into planning her own wedding while still clearly loving weddings and planning them. Maybe they were an arranged marriage, or maybe the relationship started without love. I mean she even said that love is "cultivated," but now she is experiencing the love that Tsuzuki talked about. Nonetheless, learning how one feels is an endless process, and she has learned what she wants, at least to some degree.
I'm honestly kind of shocked how cunning yet hands off Tsuzuki has been with Mitsuki and their love. She has experience and understanding, and she also just gives opportunities. In the end, the choice fell on Mitsuki. Where things go from here will determine how I truly feel about this series, I believe.