I think it's funny to imagine a meeting between the four girls.
If this happened, Miyagi would leave immediately, Adachi would spend the whole time talking only to Shimamura, and Sendai/Shimamura would spend the whole time having a warm friendly engaging conversation that neither of them gives a single flying fuck about. Like they would promise to be bff forever, and fully intend to never see each other again.
I hadn't really thought about it, but you're really right, this series has a lot of really interesting parallels to AdaShima. Like if in AdaShima, both girls were yandere perverts /j.
I sort of just see Adachi as a kinda sad cute puppy, but Miyagi has all these spiky edges because she resents Sendai for being popular and acting above it all etc. But also, Miyagi is also just a kinda sad cute kitten. Shimamura and Sendai both feel really responsible for the feelings of others, in a way that makes them keep people at arms length in case those people become burdens. But in Sendai's case, that has made her into this lonely person who wants to connect (with Miyagi only, but you know, baby steps). That's really the key difference I think: Shimamura sees Adachi as a burden, but Sendai thinks the opposite of Miyagi.