To what extent is the hadith sound which indicates that hidden evil deeds nullify good deeds on the Day of Judgment, and is there anything greater than repentance to confront past sins?

Righteous deeds are not invalidated when a person violates prohibitions in private. However, great harm befalls one who displays righteousness while concealing evil deeds. Sincere repentance erases sins, and the penitent is freed from their consequences. Such a person feels remorse for their transgressions and strives to make up for lost acts of obedience.