Does semen discharge due to imagination and deliberate pressure on the penis invalidate the fast, and does it require making up the fast and expiation?

If the discharge was madhiy, then your fast is valid, and no expiation is due from you. However, a Muslim should protect his fast from anything that diminishes its reward and should not occupy himself with practices that are from the wiles and whispers of Satan. He should seek refuge in Allah Almighty and rely upon Him, for Allah Almighty said: ﴿So when you recite the Qur’an, seek refuge in Allah from Satan, the expelled [from His mercy]. Indeed, there is no authority for him over those who believe and rely upon their Lord. His authority is only over those who take him as an ally and those who are polytheists by him.﴾ [An-Nahl: 98-100]. One should utilize time for what is beneficial, such as reciting the Qur’an and hastening to marry for those who are able, for the Prophet, peace and blessings be upon him, said: "O young people, whoever among you can afford to marry, let him marry, for it is more lowering to the gaze and more guarding of the private parts. And whoever cannot afford it, let him fast, for it will be a shield for him."