How does one return to faith after abandoning prayer and religious duties for over a year and a half due to obsessive thoughts, and is it sufficient to pronounce the Shahadatayn and perform ghusl?

Abandoning prayer is one of the greatest sins, and a Muslim has no excuse for abandoning it as long as their mind is sound. Sincere repentance and adherence to prayer are obligatory. According to the majority of scholars, it is not necessary to pronounce the Two Testimonies (Shahadatayn) or perform ghusl (ritual bath) for abandoning prayer.

Repentance for shortcomings in all other obligatory duties and adherence to them are also obligatory.

The cure for obsessive thoughts (waswas) is to disregard them and not pay them any attention.