To all the people constantly asking "but why did they break up tho" each and every chapter:
I'd argue we have the reason they broke up already. Using all the hints from chapter 1 onward, it's clear that Ritsu is an anxious person who lacks self-esteem and is awkward and prone to overthinking, and his bookish nature combined with all of that pushes him toward the outside of the school's social circles. Tsuzura-senpai has always been popular among her peers. The two of them inhabit disparate "social strata" within the hierarchy of the school as a result, and them dating is a crossing of those boundaries in the eyes of the students who care about such things—which, ostensibly, includes Ritsu himself.
So when Ritsu started overhearing gossip from other classmates about his relationship with Tsuzura, he got into his own head, worried that he'd harm his senpai's social standing, and (perhaps) because she also was going to be graduating up to high school, he broke up with her, so as not make things more "socially difficult" for her going forward. I don't think her graduating was the deciding factor, but I'm willing to bet it was an additional argument Ritsu used, and perhaps even made to his senpai, to push for the breakup.
But at the base of it all—he considered himself not worthy (reinforced by what he was overhearing from others), and made the decision on his own. Tsuzura clearly accepted the breakup, though her behavior strongly implies she didn't agree with it, then or now.
Now that they're in the same school again, Tsuzura has taken the opportunity to approach and to pursue Ritsu once more, because she still likes him (as she always did), and wants to date him again (because she, likely, never wanted to break up in the first place). She doesn't care about all the social nonsense that Ritsu was so hung up on; she just likes Ritsu, and that's enough for her (as should be the case, of course).
So this is a story of Ritsu needing to learn to trust Tsuzura's feelings, and to trust that he is good enough to date her, and that he doesn't have to give in to social pressure from others. Tsuzura likes him; other kids' opinions don't factor into that, and he shouldn't let them dictate what he and Tsuzura do.
As far as Tsuzura's behavior and...I guess "tactics" to get Ritsu to notice her again, I think that's more just her personality than anything. But she is also showing real, authentic parts of herself to him, and we also get the "real Tsuzura" when we see her interacting with her friends (or with her sister in this chapter), and we get the truth behind whatever it is she's doing while teasing Ritsu. Like when it turns out she still uses the same scented wipes he had given her, or keeps using similar supplies because they remind her of her time with Ritsu, or how she dolled herself up prior to video-calling Ritsu in this chapter. She's putting in effort, and is showing that she still likes things that remind her of her ex-boyfriend, and she's also constantly reminding Ritsu of things from their past, that even he has started to recontextualize chapter by chapter. She wants to date him, and never wanted to break up, and is showing her interest (in her teasing way) and putting Ritsu into situations where he's advancing their rekindling of romance—sometimes knowingly, mostly unknowlingly—with the expectation that he'll come to realize what he did wrong before, and that he'll grow up enough in the process to actually do what makes him happy this time around.
TL;DR — Ritsu broke up with Tsuzura because he has self-image issues and thought he wasn't good enough. Tsuzura never thought that, still likes him, and is now going after him again in a way that pushes him to reflect on their past and present and grow toward having the confidence and understanding that will make a second chance work out for them both.