The MC irks me sometimes idk why

maybe because you can't self-insert, MC got his own personality and very open about his opinion and beliefs
please stay on those blank slate OP mc so you can easily self-insert, this story is not for you.
additionally, the story and world building is good and author seems to have a real story to tell (the building of Tang Clan feared by murim warriors), they are not writing to cater to people who wanna fantasize on being a godlike character