Yep, the story was a mess before he showed up, but there was nice rhythm to it, now it's just a mess.
I was sure he will be used only to bring Sami and ml together and that's it, but now he is still in every chapter bringing nothing new to the table, like you said he is selfish af, and while he cares about her, he will still disregard her wishes, and will put what he wants in the first place.
I have no clue what the author is thinking tbh, and if by the end he will end up with Sami then I will probably never touch this story ever again, because since he first popped out I couldn't stand him.
I would rather see more of the original trio finding their place in this weird relationship.
Still I hope author will not change their relationship too much, I like Sami with pyojun too much, and I'm 95% sure he will be the endgame, but that's another reason why I think Tabin is basically useless to overall story right now.
Chan on the other hand was almost forgot by the author, and he is actually an interesting character in comparison to tabin.
I'm gonna be real I think Tabin really contributed to the story degrading. He literally adds nothing to her family plot or makes any significant progress in his relationship with Sami. Every scene he's pushing his luck, pushing her boundaries, and is literally the same before and after her change.
Everyone else from her past at least respects her decision or gives her space, but he just barrels through like he deserves to win at the end. He SUCKS as a character and a love interest but the author keeps pushing him as someone we should care about or an actual "threat" to her and pyujin's relationship.
This was coupled with the fact that MC, despite being straight forward and steadfast in what she wants, was never straight with him and was just hoping he'd take a hint and just be her friend. So they both knew he's pushing her boundaries, but they're both ignoring it. And then he CONTINUES to push her boundaries after the rejection, so he's still sucks. We know every time we see this bitch on screen it's gonna be the same stalkerish, pushy, internal whinining "but I always loved you" ass scene and it's ANNOYING and adds NOTHING.
He's getting more screen time than the ACTUAL second love interest that ACTUALLY cares about her Chan. We literally have to sit through his shitty ass scenes while Chan continues to get ZERO RESOLVE. He's actually replacing Chan as a main character and THAT SUCKS FUCK THIS GUY.