Does not performing ghusl after major ritual impurity affect the validity of the fast? And must the days that were fasted in this state be made up for? And must ghusl be performed again when in doubt about not having performed it? And is ghusl accepted if it involves showing off?

Not performing ghusl (ritual purification) after janabah (state of ritual impurity) does not invalidate the fast, but it is a prerequisite for the validity of prayer. Whoever is certain of janabah but doubts whether they have performed ghusl, and then prays, must perform ghusl and repeat the prayer, according to the majority of scholars. However, Ibn Taymiyyah holds that repeating the prayer is not obligatory in this case. Since you suffer from waswas (obsessive whispering), you may follow Ibn Taymiyyah's opinion for ease. As for your statement that you perform ghusl for other than Allah, this is a satanic whisper intended to make you abandon ghusl. Therefore, you should perform ghusl normally and disregard these whispers.