What is the correct way to perform ghusl (ritual bath) after major ritual impurity (janabah), and what is the ruling on acts of worship performed incorrectly due to obsessive thoughts (waswas) about purity?

It is sufficient to generalize water over the entire body with the intention for ghusl (ritual bath). You have already poured water over your head three times, which is enough, and there is no need to repeat it. Saying "Bismillah" (in the name of Allah) before ghusl is recommended, not obligatory. We advise you to make du'a (supplication) and turn away from the whisperings concerning ghusl.