I'm not sure what you're saying 100%, but from what I limitedly understand, let me try to answer:
Yes Taku started off seeming like he'll "save" Haesoo. But then we readers slowly realize, that he's not there to save Haesoo at all and is actually manipulative and doing this for his own selfish wants/desires. And that's reality. In life there are no princes there to save you from your toxic relationships lol. In my opinion, the authors did this to subvert that second lead "saviour" trope.
Also... Taku unintentionally made Haesoo realize that he couldn't let go of Joowon. There were multiple points to this: whenever Taku and Haesoo did regular "couple" stuff together, Haesoo realized that these were similar to what he had with Joowon and thought, "ah maybe my relationship with joowon was more REAL and normal than I thought." Also the exhibition was a key point. Taku used Haesoo's heartbreak after his seperation with Joowon as a theme for his project. He said word for word in the manhwa "I knew he was hurt/upset, but I just wanted to see more". Then cue to chapter 93/94 when Haesoo cries at his photos of the exhibition. Haesoo knew he was thinking of Joowon during those shoots, and he says out loud: the thought of leaving Joowon made him so sad that he realized he can't let it happen.
So in a way, Haesoo's personal issues of struggling to accept his feelings and relationship with Joowon was actually fixed cause of Taku. Taku didn't know it, but he made Haesoo want to confess to Joowon and stop being so cowardly with his true feelings. Haesoo realize he feared losing Joowon MORE THAN getting hurt by Joowon's rejection (since Haesoo thought Joowon never genuinely loved him and this was a whole miscommunication btwn them)
You know what I was okay with it then I started reading comments and then I started to get really bothered lol. Like they really portrayed( or at least to me) that Taku was gonna help in some way or be better for Haesoo. They delved so deep into him where it really was surrounding him only for it to be a eye opener (I guess?) As to why he still loves Joowon. Like all that and for what and its true none of Haesoos personal issues were fixed only person who changed was Taku. He's had relationships before and even told them to Joowon how is this one different that he realized that he only wanted joowon. Qas it because there was potentially a relationship out of this? Did he actually have some feeling for Taku? Or was it simply because he had some contact with Joowon as well? I hope this doesn't sound confusing and if it does well that's how confusing it is to me lol