If a revert previously prayed and fasted only occasionally, missed many Ramadan fasts, and does not know the exact number but very few times fasted, what is required after sincere repentance? How should the fasts be made up, and may one combine qada with Sunnah fasts after this Ramadan?

There are many sayings for the people of ilm, some necessitate that you make them up. Yet some say that by being heedless with the salah, you have abandoned it, thus left Islam. Therefore you do not make up anything and you sincerely repent to Allah and increase in seeking nearness to Him and in voluntary prayers and fasting.