Responses
Plus let's be real the artist do draw their character pretty similarly. They're built similarly. Plus yes I know they had their hair colored changed but the confusing part was who's hair color had this color when they were young. Reading this while it was ongoing was also a facter because I had to refresh my memory when it updates.

IMO, why people can't distinguish the characters:
1) people don't pay attention to details, all these people are different and I had no issue with knowing who is who.
2) I think *the biggest reason is there is there is no usual childish tiny uke/ big mean seme trope*. They are all adult men and artist portrays them as adult men, including their faces. It's adult story with adult artstyle and it shows that this is very rare in manga/manhwa scene (I mean there's plenty of stories like this, just they are not easy read slop and are rarely popular.)
3) They are all average handsome adult men, so they are kinda similar, but not identical because handsome features need to be specific to be considered handsome.
4) artist let's them grow and change, ie dye the hair, grow tall etc . It's so rare in comics, that when it happens, people are surprised and confused, because woojin didn't have his hair blond and spiked in high school and choi isn't a natural redhead (he's Korean, so duh).
I personally think that the artist could work some on drawing faces from profile view and could lean slightly into caricature, to make the differences bigger, but genuinely I think when it comes to art, I think that amid clip studio 3d slop this one shines through as the artist has actual drawing skills and doesn't do another copy catted manga-big-eyes thing, but goes their own path. I only wish they had proper time to draw and produce high quality they are truly capable of.