But two characters that potentially mean new deep connections and a chance for growth and introspection for Aya and Mitsuki will make for a good upcoming arc, I think.
Though I do agree that it's entirely realistic for high school couples to break up and to form new connections and such, I don't actually see how either Aya or Mitsuki could form any deep or meaningful connections with either Shu or Rinko.
Shu is more likely to just use Aya as a means to an end of getting over her own shortcomings and the pain in her knee and beginning to form actual friendship that isn't one sided or that could feel obligatory in nature. Aya has nothing in common that we've seen thus far.
Rinko on the other hand, she is absolutely just using Mitsuki as a way to boost her own confidence. She sees Mitsuki as what she could be, and more or less idolizes her. It's likely that's what caused the infatuation. Because of this, Mitsuki would also not be able to form a deep meaningful relationship with her.
All that said, I don't believe either of them have any actual ill intentions regarding their feelings towards Aya and Mitsuki. It's very clear that neither of them want to get between the girls, and that their feelings are just overwhelming them because one of them knows how she feels and really doesn't want to get in the way while staying close, and the other seems to be giving off the "I'm not gay!" vibes, where she has feelings for Aya but doesn't actually know what they are or how to feel about them yet. It just feels like what would be more likely to happen would be that either one or the other confesses in some way, and either Aya or Mitsuki (Whichever isn't confessed to) overhears it. It causes some mild drama and doubt with them (Especially after Mitsuki's whole mood shift and Aya clearly recognizing it. Mitsuki would have the self doubt and Aya would... Most likely not lol), but ultimately doesn't do anything to their relationship. Aya has grown to the point of knowing what she wants and being more likely to fight for it, and this could even end with Mitsuki deciding she doesn't need a band and getting back into the swing of things.