It's not about the feelings he had for Mihee. It's about their relationship and what they went though. Saying goodbye to her was like officially closing the chapter of his past and move on, but that didn't work and now Suchan has to see and fight a person he cared for
I feel like I should've put (jk) somewhere in my comment. Thought I made the comment as sarcastic as I can
And yeah, I agree with all your points. Not only is he just closing the chapter of his past but also his chapter of being human. Iirc before the hiatus or season break (whatever it was) Suchan was still tempted to return to his human life. Sure the series might end with everyone turning back to human but that's that and this is this.
Babes you already have a loving girlfriend, ditch the pinky and stay with her!! Sick of these love triangle-esque tropes