I recommend an ancient romance novel called "Yongzheng's Mistress". The title is the same as the one mentioned in the question. The female protagonist was an ancient flower demon. After her rebirth, she married Yongzheng as a concubine in order to improve her cultivation. The story mainly told her life in the Inner Residence and the palace fights with the elites. I hope you like this fairy's recommendation, Muah ~๐