It sucks because Dohu is just...autistic. like actually. He didn't really view it as using him for ulterior motives because he was very upfront about what he wanted from him (for him to be himself) and then the moment he realized their relationship had gotten deeper and that would be an invasion of privacy, he stopped. He knows retrospectively that it could be perceived as being two-faced from other people, and I think he's so used to being "abnormal" that he just accepts that what he does is going to be perceived however other people please and he's in the wrong...but if he explained himself properly and actually communicated how things played out and that he genuinely didn't know what he was doing was wrong + how he stopped the moment he heard it was bad, a lot of this would have been smoothed over.
I do think that would help but unfortunately wouldn't solve the root problem because Juheon's primary issue was that even when Dohu realized what he was doing was offensive and stopped, he never honestly admitted his actions. To Dohu, it was all water under the bridge and I can see why he thought that -- their feelings were now mutual and revealing his original true intentions would have soured their relationship. But to Juheon who really prioritizes transparency, his argument is that Dohu should have been honest the moment he realized his actions were wrong instead of trying to keep it hush.
I personally don't want that type of character development from Dohu, because Dohu has always been an empathetic character, like how he chose to stay with the little kid in the early chapter. The problem is Dohu is naturally slow at understanding certain things. He has attempted to understand others all his life, even going as far as to be a writer, but still couldn't due to his natural constitution. You simply can't ask the blind to suddenly see.
Dohu recognizes this, hence why he doesn't argue whenever people tell him he is the problem. I would hate if the story goes to prove that everything that goes wrong is automatically Dohu's fault, and that Dohu is doomed to never has anyone to accept him for who he is. For this relationship to work, it cannot be just Dohu's one sidedly try to solve. Juheon himself has a lot of growing to do.
To me, his helping the kid showed that Dohu was a genuinely kind person and would do things for others even without personal gain -- thats why it made such a genuine shift in how Juheon viewed Dohu. I would say that Dohu's fatal flaw is more that he can be self-centered, not that he can be slow sometimes. It's not the beginning of their relationship that was the real offense, bc he didn't actively try to mislead the ML, but it was when he realized they had mutual feelings and still wanted to forego telling the ML, despite knowing the latter's thoughts on being used. Even now, discounting the shared work places, he continues to insert himself into Juheon's life even while the latter is expressing great discomfort.
I agree that Juheon also has areas to grow -- his fatal flaw is that he is, frankly, a coward. Rather than dealing with things upfront, his preference is to hide away, run, and stew in it until it boils over. If he had been more confrontational, his life in many ways probably would be better. But i think the main difference is that Juheon's flaw is a detriment to himself, something that eats away from inside. On the flip side, Dohu's flaw directly impacted Juheon.
Now this doesn't mean Dohu is doomed, bc all he really needs to learn is to be honest next time when he's in the wrong. No one is asking him to suddenly have high EQ or even always know what is right vs wrong, just honesty. Realistically, this is an easy lesson for Dohu to learn if his partner was someone more confrontational(aka not Juheon) and dating in the future would actually come much easier.
Also, narratively, i think it makes sense that Dohu's mistake is the root problem. In many bl webtoons, we usually see a perfectly sweet MC who can do no wrong and a strong assertive ML that basically inserts himself into the MC's life. This story is in many ways, a reversal. We have a bold imperfect MC who has always done the chasing, and a cowardly ML who needed emotional support. This reversal of common tropes is what drew a lot of ppl to this story and is what makes the MC so compelling, bc he's a dynamic character with pros and cons that develop as the story does -- not a mary sue.
I think the issue I have here with your interpretation is that you placed most burden of the problem in this relationship on Dohu. If you want to say Dohu is self centered, what does that make Juheon who abandoned Dohu without any explanation for 4 years? They are both selfish. It's not born out of malice, but deep traumatic people who have very flawed coping mechanism. Dohu chooses extreme apathy and ignores most of his problem, while Juheon chooses to close himself off and would hurt others first to protect himself.
For example, you seem to think that Dohu continuously inserts himself in Juheon's life as a sign of Dohu's lack of empathy, but I see it as character development on Dohu's side. Throughout the series, Dohu has always viewed relationship as annoying and he believed that none of his relationship would ever last, Juheon included, so he has always mentally prepared himself for eventual separation. This line of thinking is something that has been instilled to him by everyone around him, in particularly Gitae who mocked him, saying how could anyone ever stand being with him. Dohu has resigned to the fact that Juheon will one day hate and leave him.
On that final day before Juheon left for good, Juheon came to Dohu crying that he didn't care about Dohu lying to him, but still hoped for Dohu to love him. Dohu before that was ready to let go of their relationship, believing that was what Juheon wanted, but after Juheon left, Dohu came to realize that what Juheon truly wanted. Juheon wanted Dohu to not let him go, but because Dohu was so slow at catching this, he was too late. This is why Dohu was left to regret for the next 4 years, and determined to never let Juheon go anymore when they met again.
Perhaps Dohu's method of wanting to insert himself in Juheon's life again is too aggressive and can be seen as invasive, but with Juheon's outright refusing to listen to Dohu or go as far as asking Dohu to quit his job, there is really not much Dohu can do. Dohu has been working very hard to understand and communicate with Juheon, but it's all for naught if Juheon himself refuses to open himself up. At this point of the story, it's not Dohu that needs growing up. It's Juheon. This time, Dohu is as straightforward and as honest as he can get. It's Juheon that has been lying over and over. Pretending not to know Dohu, then claiming not to care about Dohu while still holding a memento of their relationship. The one that needs to be more honest now is Juheon.
Reading this, I can literally feel Juheon's irritation towards Dohu. While Dohu is a very likeable and interesting(not perfect which makes him better) MC, I do get why Juheon is irritated. His biggest fear is being used by someone who's two-faced(aunt, that sunbae in college, that other sunbae that gave answers, etc.). He falls for Dohu precisely bc he presents as someone abnormally honest and upfront, and per his words, liked Juheon for who he is and no other reason.
So to find out that the foundation of your relationship is built on exactly what you fear most(being used for dishonest purposes), by someone who you trusted, it not only ruins the trust you have in the other person, but also yourself. It would quite frankly, make a person mistrust their own judgment.
I can't really tell how this story is going to progress. If it's just Dohu convincing Juheon of his honest affections, it honestly would feel repetitive of season one. I actually feel like Dohu needs a redemption arc, bc while he is funny and upfront, he does have a tendency to push himself onto others in a way that's unwelcome.His character flaw is that he can be quite selfish which narratively, makes for great plot and character arcs. Personally, I hope we get to a point where he can learn to be a bit more empathetic, and where Juheon can learn to be more confrontational.