Is it prescribed for a Muslim whose sins have multiplied and who has forgotten their details, and for whom it is possible that some of them reached the degree of apostasy, to utter the Two Testimonies as a precaution?

The causes of apostasy are clarified in Fatwa no. 146893. The fundamental principle is the persistence of Islam, and a person is not judged to have left it merely by doubt, so you must abandon these whispers (waswas). Uttering the two Shahadas (declarations of faith) is among the best deeds. If you have forgotten your sins, do not recall them. A general repentance, through which you intend to cease everything that displeases Allah and resolve to adhere steadfastly to His law, is sufficient for you. For more details, please refer to Fatwa no. 112047.