Is it permissible for a person who used to sing, and then became committed to religious observance, to sing to himself or to his wife in the future, knowing that he recites the Quran, masters its recitation (tajwid), and regularly observes tasbih and istighfar?

All praise is due to Allah for bestowing guidance upon you, and we ask Him, the Almighty, to keep you steadfast upon it. The ruling on singing to oneself and in front of others is explained in Fatwa no. 143242, which includes the permissibility of some forms of chanting (huda') and similar activities, to ease the soul during difficult tasks. The fundamental principle is that a servant's singing should be with the Quran. Please refer to Fatwa no. 103176.