The more we learn about Kiyoshirou's busted up family, the more this whole "I am going abroad to pursue my aspirations" (whatever it is those are) thing is becoming kind of disconcerting. I mean, on one hand it's his life and goals, he can't change who he is and you can't really blame him for having lofty goals no differently than the rest of his family (though, I get the feeling his brother doesn't really like that lifestyle but feels obliged to follow it regardless of his own feelings) but even his mother seems to be pulling double duty here to highlight just how ridiculous this looks from the opposite side.
Of course, part of the problem is that this book illustrates how fickle love can be and as charming as it is that they've found something special in each other despite being so different, but the fact that they're so different that Koharu has no chance of catching up with or even being in a similar position to him, to the extent that there's no discernible path for her to even follow him just highlights the problem. It'll just be him leaving to pursue his aspirations leaving her behind and if she wants to continue being 'with him', she'll have to put her life on hold waiting for him for half a decade or more. And even if he does return (or she goes to him) there's no guarantee that he wont end up being just like his mother with his attention entirely focused on his work to the neglect of everything else. Yet, Koharu is so timid, lacking experience and self-confidence that coupled with her growing infatuation for him... she's at risk of throwing everything away for the sake of being in love with a guy that has all but told her that he will neglect her almost as badly as his own mother neglected him. And she's basically already started stockholming herself into accepting that.
That said, I am not saying Kiyoshirou is a bad guy for being fixated on his aspirations, but it strikes me as selfish for him to allow this to continue. He knows what kind of future he's pursuing and it isn't one where he sacrifices some of his potential for the sake of love, but one where he's going to repeat the same mistakes his mother made, that have made him and his brother resent their family. And more than that, he's smart enough to see that Koharu is becoming increasingly co-dependent on him and she's the sort of person that needs to be with someone that can be attentive, caring, and available. And Kiyoshirou is two of those things, but if he's going to choose work over being the kind of man the woman he loves needs, then he's only choosing to hurt her by proxy. Even though their relationship has been charming and delightful, the best thing he can do for her now is to end things so he doesn't hurt her worse later and she can take what she gained from their relationship and work on developing confidence, experience, and become her own person again away from him.
I haven't looked into the raws and I don't intend to, but knowing the book is done, the only other way I can picture this book ending in a positive way is if Kiyoshirou realizes that love and family is more important than being a slave to your work. Otherwise a breakup end is the only good way both of them can truly live their best lives. Fundamentally, it's no different than being married to someone who's work has them traveling all year long and we know how those work out (hint: they don't). Some women can handle it, but most can't and the more you take that for granted the more likely it's going to eventually end in tears.