Please bless the people with Sanfan opinions!
Anon, when I first read this ask, all I could think about is how I don't think about Sanfan at all. Which inevitably got me thinking about Sanfan lol (← girl who thinks about women all day).
We don't have a lot on Sanfan, she's practically a LN 13-only character, but she's introduced so she can mirror two characters we know much better: Yao and En'en.
Sanfan ran away from home to escape a forced marriage arranged by her wealthy merchant parents and has a real talent and commercial knowly or else Lakan wouldn't have given her time of the day. Just like Yao, Sanfan didn't want her family to decide her future for her nor be reduced to a marriageable asset: she wanted to make those choices for herself and did the best she could in her circumstances.
Another thing she has in common with Yao isn't just her love for Lahan, but why they're attracted to him at all: Lahan doesn't really care about gender, only skill, so he wouldn't understimate a person just because they're a woman. Sanfan is even more explicit than Yao when giving out this reason, she says in LN 13 that Lahan gave her an opportunity to be independent and that very few men think like he does. For someone about to be forcefully married just because they're woman, her family completely disregarding how good her commercial skills are, having someone actually appreciate and treat you like you're competent must have been pretty novel to Sanfan and she was understandingly charmed.
Maomao and En'en, no matter how much they understand that Lahan's pragmatism and disregard for gender might be the reason he's attractive to women, can't really get it because they were never in Sanfan and Yao's position (Maomao was raised to be a skilled apothecary by Luomen so she could support herself and En'en expects to serve Yao until she dies). Sanfan and Yao are so painfully similar, with lots of common experiences and ideals, but they both want Li's only proto-feminist so they wouldn't ever realize that.
Hell, Sanfan even resents Yao bc she thinks Yao is unsubtle and prepotent and trying to force herself into being Lahan's wife because she isn't good enough to do it by other means. She thinks Yao is a "wet behind the ears girl" and thus can't respect Yao enough to let her take her shot nor to try to sympathize with her.
Honestly, compared to Yao, Sanfan goes even further, she breaks social norms that Yao wouldn't ever dream of. Sanfan wears masculine clothes so she would be mistaken as a man and respected, something especially important to her as she mainly works doing business for the La Clan. Sanfan doesn't seem to believe in marriage as something for monogamous love, but really just another thing nobles check off from a list. She is older and more mature than Yao and understands how much she can push. You could even say Sanfan gets Lahan better than anyone and loves him with a full view of his flaws.
Since she's kinda of a more mature Yao, she seems like a good Lahan endgame, right?
But here's the deal: Sanfan is also a mirror of En'en. She's a girl that was handed a life-changing opportunity in her time of need and gave her all to show they weren't wrong to put their trust in her (like En'en and her brother's situation with the Lu family). Like En'en, Sanfan cultivated a deep loyalty and love for her master and they both don't dream of stepping above their station, despite how desperately in love they are: En'en believes Yao should have a husband and Sanfan would be happy to be Lahan's mistress. As long as the respective spouses fit their carefully curated criteria, of course.
Love and loyalty are good, don't get me wrong, but I did mention before in another rant of mine that En'en needs to let go of her obsession with Yao to grow as a person and survive the Great Enyao Rupture bc there's more in life than just being obsessed with one person. And I believe that, if Sanfan is meant to mirror her, than she also needs to grow past her obsession with Lahan. We barely got anything about Sanfan, but she's being solely focused on Lahan, to the detriment of even her own work (Lahan himself says he'd rather her do actual work than drive him around), her main selling point to the La Clan.
If En'en is all about Yao, then Sanfan is all about Lahan, and they're going to have to face reality when Lahan/Yao inevitably happens and things don't turn out the way their planned for their beloveds.
Sure, Sanfan could stay and be Lahan's mistress (which I don't think Yao would allow tbh, I think Lahan will change a bit to finally settled down with her) and I could even see it be played up for comedy, but there could be so much more for her. If Sanfan is a character introduced by sensei to hold a mirror to both sides of enyao and their relationship to Lahan, I believe she can get a satisfying resolution when this storyline is tied up and get something for herself beyond her love and devotion for a married person, just like I'm hoping En'en will.
Sanfan and En'en need to travel together presenting as a married couple (butch Sanfan and fem En'en yay) and [redacted] and [redacted] and even [redacted] as a treat so they can heal and grow as people :)