I wonder if that's just what this is, though: maybe she's the straw that'll break the camel's back. This is the third break-up Hiromi's been through in, what... months? Weeks? He's either going to keep making the same mistakes for a while (or maybe forever, if Sasuga's feeling a bit sadistic) until he does eventually learn, or this will be what causes a significant change in how he views his dream of a perfect family and which kinds of girls he pursues from here on out. Either way, whether it happens immediately or if he's going to make even more, even messier mistakes that have lasting consequences, I'm predicting this break-up is going to hit him MUCH harder than the last one with Lily. Hiromi has a very idealistic view of relationships currently, and that MUST break down and something much more realistic (and hopefully healthy) needs to take its place if we want some sort of good ending.
He had a big arc with Anju that, in my view, ended in him helping her see that she needed to grow past the confined view of romance and love and relationships that were instilled in her via her mother. That this happened in the midst of him "going backwards" by reconnecting with a childhood friend/crush with the intention of moving out & in with her and pursuing marriage, makes me think that this whole arc twisting Nonoka and Anju together is foreshadowing for Hiromi specifically.
He's definitely still operating under this incredibly idealistic—arguably naive or at even childish, maybe—view of what "a relationship" entails. Marriage, kids, a house with room for all of it. That's like the pictures 1st graders draw in class complete with the smiling sun in the corner of the page. Hiromi's lived experiences have shown him that such fantasies are largely just that for a lot of people much of the time. Somehow the most put-together person in Yukari feels like she'd be the healthiest partner for Hiromi, but as someone else said elsewhere in the thread, they sit at cross purposes, and Yukari's figured her life out already and is perfectly happy having no one and nothing tying her down.
If I had to guess, and if I were assuming Sasuga Kei is using some "established" form of character-centric storytelling format, that we're actually hitting the point where Hiromi starts sinking into a darker place. I don't know if he'll actually get
angry, but this is three failed relationships, after he'd turned down Anju, Mio, and Tsubasa to where the latter has gone and gotten herself a rebound that she's clearly second-guessing. Four of the Five housemates want to actively pursue him at this point to some degree, and Hiromi might swear off "committed relationships" at this point and try the impromptu route this go around. A sort of "this isn't working, now for something completely different".
I actually think we might see Tsubasa come back in. She also just had a "meet the parents" moment where the matriarch of her family/company just told her "who's this scrub? Where'd that nice [Hiromi] feller go?" and she's been regretting it since I bet the day she got drunk and slept with her kouhai. She's under increasing pressure to do something about this succession business, and I kinda wonder if she's gonna get drunk and push Hiromi down (or even
pretend to be drunk while doing it for plausible deniability), and maybe Hiromi will just lean into it because "ah, fuck it who cares".
Which I think would piss off Anju especially, but also get Lily re-involved, and perhaps even Mio. (oh, plus Tsubasa's boyfriend.) That would certainly make stuff way messier, but it could also be a way to sink Hiromi's character into the narrative muck of complete chaos and uncertainty as to who he is and what he actually wants, essentially annihilating his ego and forcing him to rebuild himself from the ground up as a person rising from the wreckage.
Basically, I think Hiromi might have to "destroy" himself and then remake himself. This feels like a prime point for him to be fed up, swear off anything meaningful, and sink into a bad place of relative debauchery before getting his feet back under himself—maybe once again with the help of Yukari, or maybe even with the aid of the one pupil he had who he'd been "tutoring" toward being a better person in the dating scene. There are people who are out there who could serve as guides and helpers to him, but I think he needs something that'll fully dissolve the rigid perspectives he's clinging to before anything constructive can be accomplished.
All the messy drama that involves mutliple sharemates in a sexual capacity would just be prime "Sasuga Kei Soap Opera" material along the way.