Is imagining illicit intercourse with a woman one was previously guilty of harassing, followed by masturbation, considered a major or minor sin, given the frequent seeking of forgiveness, repentance, and avoidance of this woman?

Sexual fantasies, and the act of deliberately summoning them and taking pleasure in them, are not permissible. However, if they overwhelm a person without being actively sought, there is no accountability for them, as they are considered minor sins. One must repent to Allah for them by feeling remorse, seeking forgiveness, immediately ceasing the act, and resolving never to return to it.