i stg dont put a triangle in...the material is heavy enough. dont do this...its gonna be too dramatic. there are so many fanfics that attempt to tackle the topic or make light of it here. it's quite sad and disheartening. like ive gone through most of the twice ff that deal with depression and uality and few do it justice. i can count those on one hand. to put it bluntly, it really ruins those works. yours actually looks like it can be a solid fic without a triangle. making nayeon come to terms with her uality if you go that route can be done separately. i would rather have chae and nayeon be a shoulder to lean on for each other. it sends a better message. you can still add arcs for nayeon too and make a spin off if you leave out a triangle.
ps: personally i think momo and nayeon is best. im namo trash haha