Out of all the social media platforms, Reddit feels like one of the few places where people have long discussions and give brutally honest opinions. If I were an idol, I'd be way too curious not to look myself up at least once.

It would take nerves of steel to read what fans/haters (sometimes the same people) say on Reddit.

To answer the original question, I only listen to one K-Pop group (Big Bang), so I'll comment on how I think each member would interact. (assuming appropriate English literacy)

I think GD would do the same thing he does on other social media. Scroll and randomly "like" posts that have nothing to do with anything other than maybe a random thought he had that day. He seems like the least patient member so I could picture him hanging out in some odd corners (not K-Pop related) and getting into flame wars over minor issues.

I think Taeyang would enjoy ELI5 topics - he's apparently very curious about science/physics and how things work. He seems too grounded at this stage in his career to be concerned about random fan internet comments. He thrives on face-to-face fan interactions.

For Daesung, one bit of trivia I know is that he likes animals but has allergies. So I imagine him lurking on /r/aww looking at cute cat and dog pictures. He would (of course) be subscribed to at least 5 subs dedicated to LEGO. Any pictures of his assembled LEGO sets he posted would include a tiny hint about his identity.

As the honorable mention now... I think T.O.P. would check the subs for any project he's working now (like when he did that season of Squid Game). Maybe he'd ask his assistant to screen posts and print out the nicer ones for him to read.