I think he was also apart of the distribution of the h-100 drugs unknowingly. Cause I remember a line where he said he's also at fault for turning Jaeyoung into a person who can't feel emotions anymore.
I think it's all rooting from Jae, just my hypothesis. Someone debunk that it's not because Seungmin searched about Jae when they first sleep together, just slept. But, if you think about it in early chapters there's a convo with the Bar Manager about making the strobe lights intense baiting those who sells the drug, and we all see that realization Seungmin had when he realized and connect the dots, then he's linked to Jae's doctor and I think they're tracking down the source of the drug to find a way to cure Jae, well like that but who knows? in the last season it's indicated that Seungmin will now navigate and steer Jae's emotion, like a captain holding his ship. (indicated for me, idk to others)
The Doctor = finding ways to make Jae feel emotion again, helping and researching about the drug and finding ways to prevent it? or navigate around the effects of the drug.
Seungmin, I just think he have a big fat crush on Jae, like he said music is the only thing that makes Jae respond so he used the music as way to be with him. When I said it's all rooting from Jae, all I can think of is that if they rooted out about the drug then what happened to baby Jae will come out too, or the other way around. A big win to the Bar Manager, the doctor, and Seungmin as they've been investigating it. Again, just my hypothesis
The more I read, the more curious I get to know what Seungmin's main job is... I just read chapter 24 again, and Seungmin's boss from the agency said, "I HEARD HE WENT ON THE SHOW JUST TO GET FAMOUS. ACTUALLY... TO BE PRECISE, DIDN'T HE SAY HE HAD TO GET FAMOUS?" It seems like Seungmin is trying to become famous so that someone will notice him. Not to mention the fact that he's got a lot of connections with people related to the drugs. On top of that, he and Jaeyoung's doctor met each other at a social event in Brazil.