Ren isn’t interesting
This is a complaint I haven’t heard very often. May I ask why you feel this way? I find him wanting to fulfill the “wishes” of his Grandfather while being pulled by the Light and possibly lunacy to be extremely compelling. And aside from that like that Kylo is a bit of a smartass and his temper was both frightening and humorous.
There are a few more things I could mention but I think he has a lot going on as a character. Certainly more than Vader in STAR WARS, Maul in TPM and Palpatine in general.
Yeah, hard to see this as a legitimate complaint, especially from someone who likes SW enough to be on this forum. Ren is by far the most complex Star Wars villain to date.