
I was so startled when he went and find a "substitute" it felt so sudden. Is that supposed to be a part of something in the future or just something that passes by?

Yes. Tamaki is one year older so when he started highschool, Shii decided it would be a good opportunity to put some distance between them and get over him since his love was unrequited.
So he pretended to be a college student and met Takumi (a real college student) via a dating app. They became sex friend and it lasted for about a year. They broke it off because Shii realized he was still in love with Tamaki and Takumi could never be a replacement even though they look similar (it's because their personalities are complete opposite too). So when he also started highschool he reunited with Tamaki and decided to be close friends again. But as we know they became something more.

You're welcome. And the reason they're still meeting up in the present is because they're still friendly. Taking to him also helps Shii because Takumi can give him advice about Tamaki since he's older and has more experience in both love and sex
But that's all it is, Shii is faithful to Tamaki. He even refused Takumi's invitation to have sex again because he only wants to have sex with Tamaki now.

Maybe their relationship did not actually progressed that much becuse it seems fast? Like its only about sex and nothing more. Im thinking the MC might ask the yakuza bout their status and he'd blantantly tell him hes not serious or something lmao thats a common predictable plot but irdk

It feels too rushed. The way minjoon suddenly accepted toma as his kid and let him call him his mom. Like wouldn't u be scared of Yakuza and try to this this is bs and run away atleast. He accepted it too easily. Daiki and minjoon has no had one proper conversation. Let alone the fact that Daiki didn't even show interest at the beggining and then suddenly he who has married and had a kid with another woman did those things with minjoon??? I find it weird. I mean minjoon is gay atleast but idk daiki and minjoons relationship progress is weird af

The novel makes their feelings way clearer https://archiveofourown.org/works/28126470/chapters/68914599

I LOVE that there are always explanations at the end of every chapter. DID is a form of defense mechanism brought by trauma. I only know that much and I am super interested in learning more. My course would also go in depth about these mentall issues and I cant wait!! Psychiatry and Psychology are very interesting for me.
Do you ever just get bored cuz the scene is too typical but concerned at the same time cuz he's getting raped. I just did.