This may be a little mean-spirited but given the widespread negative attitudes towards polyamory in fiction, I find it very satisfying that it is consistently Kashiwai's monogamy that causes problems in this series.
I wouldn't say it's "her monogamy" that's the issue, exactly, more what you said about her not being honest with her partners (or, really, herself) about what she wants.
Like, what's the endgame here? She doesn't really, truly seem comfortable with "sharing" her crush. But she's also unwilling to give it up, both because of the strength of those feelings and, likely, a sunk-cost thing. "I've already sacrificed my other relationship for this, if I give up now it'll have been for nothing."
So what happens? Mizuki learns to accept and be happy with "just" being one name among several on Tsuzuki's schedule? Tsuzuki gives up on her other relationships in order to be with Mizuki? I feel like the two of them, despite their feelings, are just fundamentally incompatible.