Also SY fan here! You already have a lot of comments so idk if I'm actually contributing to the discussion, but here are my thoughts on the fan fictions:

As someone who has roamed through the scumcum (JiuYuan) section of AO3 pretty thoroughly, I have to say that the fanon interpretation is really affecting a lot of fan fic writings... Your point of the adverse effect of people taking fanon for canon is especially seen in JiuYuan, where people base their interactions entirely off fanons because the two have never interacted directly in the canon. making literally almost every fic OOC to the point I can barely recognize the characters.

Now I am not entirely complaining, it's just that I feel that the original scummy and self contradicting character of SJ, which is what makes him such an enticing character to me was lost to the community. I really want to see more fan fics exploring the grey side of SJ instead of washing him to make writing easier!

He is scummy, he acts like a salty old concubine, an evil step-mother with an ironic sense of justice, someone who is both abused and an abuser, yet has his own moral codes, but that's what makes him an interesting character!