Kinda late for the discussion, but I feel like Sumin still had (if not now) feelings for Seju. Could be wrong of course, but it really seems she's still trapped in her past. She also seemed upset when Seju said she was going to get married. Gotta admit, I liked their development more than Sungji and Sumin.
Agreed about their better chemistry compared to Sungji. I mean for Sumin x Sungji, I feel like most of the time one of them is taking the lead and the other follows, but rarely they're walking side by side (figuratively, if you know what I mean). At first I didnt have good feelings about Seju because Sungji was just so cute I didnt want anyone like Seju to hurt her, but then it was shown time after time that Seju really respected Sumin and patiently waited for her. And they had a lot of cute moments in the past. When they're together, it seems like they understand each other, they're on the same page.
However they have been stuck in the past for too long and the scar is too deep; I think its too broken to be fixed. If this is a healing story like the authors claimed, they should only move on from each other, not towards each other again. Thats what Sungji came into the picture for.