I mostly read short fluffs which is the literal opposite of what you like lmao but I think I can pitch a recommendation or two your way
Night rather than day - Winkniel, ongoing but updates regularly so dw, this could look humour-ish at the beginning (it is funny tho) but each chapter gets more intense and honestly I love it
'idk' (and other cute ways of saying 'i like you') - this may not be chaptered but it's long, 22k words, Ongniel, they can't sort their emotions out and it's frustrating, a lot of this was super relatable and had some good quotes.
Ongniel have a lot of chaptered angst fics tbh? What is it with this pairing and suffering
Scroll down the Wanna One tag on ao3 sorted by most kudos and you'll probably find at least several things your taste
Also, you could tag "wanna one" on the thread so a wider array of people can come recommend.