Do doubts about the intention or washing certain parts after performing ghusl following masturbation necessitate re-performing ghusl?

You have done well by not paying attention to the whispers (waswas). Doubt or whispers have no effect after an action has been performed. Your ghusl is valid. We pray for your safety from waswas by ignoring them. We advise you to constantly supplicate and humble yourself to Allah, to distract yourself from these whispers, and to consult a trusted psychiatrist.