thanks for asking my advice in my comment "chapter 7"
my suggestion is, why don't you put the first romantic scene in the part where the Hwal returns the Sangun karimbit knife, n suddenly or unintentionally there is a scene that brings them close to each other, very close n makes them almost kiss, but is interrupted by mr. Kwon's arrival, who came suddenly n catch them)
N while in the car on the way to battle, they both realized each other's feelings n made a little awkward btween them.
then in the next scene, in the missing scene in episode 16 (End), the scene where Siho sees the memory of Sangun who is dying, when Sangun tries to hold Hwal (when Hwal tells about his past bad actions and Sangun says that she will help him, because Hwal has sacrificed enough and allowed thing to stay alive). u can put skinship n kiss scenes when hwal is just coming out of the old well n when ok ul tae goes to meet her last victims on the road. at that time hwal met sangun and hwal realized that his feelings had not grown recently but his feelings had been there since 1000 years ago, feelings of love for sangun were so deep. Feelings of love that are now mixed with regret because of his misunderstanding at Sangun 1000 years ago, feelings mixed with a very deep longing for Sangun. then the scene continues on the missing scene that I described earlier.
That my suggest, i hope it can help u...
But its all up to u, bcause i know u're better than me. ✌️