(this is not actually a thread fic, just me wittering on. I have not written fic in over 20 years, unlikely to start now, it's a struggle to even think of ideas, let alone write them out in a narrative format.)
I don't have any firm ideas about when Changbin & Hyunjin actually start doing more than flirting. But for maximum chaos & angst, it would be good if it happened around the time that Felix arrived in Korea (about 6 months before their reality show).