The idea of consent in the context of sex requires that the two people involved are, well, people. People who can enter into agreements with each other willingly and freely. That's why the age of consent exists in our society - to make sure that the parties entering into the agreement both have equal-ish standing and rights before the law.

From the first second of the Apothecary Diaries, it is clear that this is simply not the case in the world in which Maomao lives. She has no recourse to the law at all. She has no rights. Things can just be done to her, and there is no higher authority to which she has a right of appeal. Part of this is being a woman, part of it is being a commoner without money. Both roles objectify her, turn her into a thing to be bought and sold and disposed of. Maomao is rendered quite honestly and perceptively here as someone who was raised in this society and internalized its values. Part of her low opinion of herself comes from the circumstances in which she was raised, but most of it is a simple reaction to what she knows to be true.

And then we come to Jinshi, who is also not really a person, but in a different way. Jinshi is not a person before the law because no law could bind him. There's exactly one other being to whom he has any accountability whatsoever. For all intents and purposes, he is the law. Now, unlike Maomao, Jinshi has the blindness of privilege, so he can't see the power difference between himself and everyone else so clearly at the start (over the course of the story, he comes to understand it better).

There's no sense in which an object and the law itself can enter into a consensual relationship. And that's the root of the majority of the story's tension. The reason they haven't yet got together is that it is impossible for them to do so as equals. Maomao is still only barely capable of seeing herself as a subject of desire and agency, and Jinshi has become keenly aware of the fact that she isn't capable of refusing him. I don't know how this story will end, but that's the key problem that the resolution of the story must solve. This world can't support a romance between these two people.

The most recent light novel makes this point extremely clear. Ah-Duo represents the best possible outcome for a romance between monarch and subject in this setting. She's well-treated, she gets along well with the Emperor, and she's allowed to basically do her own thing (her own thing = collecting a house worth of assorted gender weirdos to hang out with, good for her). But she doesn't see the Emperor as a person, and he doesn't truly see her as a person either. They're not people with respect to one another. That's the fate that Jinshi and Maomao must avoid, and it can only be avoided by fundamentally changing their roles in society, or by changing society itself. They have to invent consent.

A lot of historical romance uses the fact that consent isn't a well-developed thing in most historical societies in order to enable certain fantasy scenarios to occur. Giving the heroine 'no choice' is a good way to allow the reader who empathizes with her to vicariously live out desires they would never allow themselves to choose in real life. I'm not here to critique or praise that. It clearly appeals to a lot of people, let a thousand flowers bloom, et cetera. But it's demonstrably not what The Apothecary Diaries is doing. The Apothecary Diaries is about consent.