Is it permissible to suffice with repentance for missed prayers, and for their make-up not to be obligatory, on the grounds that abandoning them constitutes disbelief, knowing that he used to pray and maintain prayer during Ramadan? And what is the ruling on his fasting on Mondays and Thursdays with the intention of making up for missed fasts?

Your duty is to emulate a scholar whom you trust, as you are a layman. The disagreement regarding the obligation to make up prayers intentionally missed is well-known. It is permissible to emulate those who do not deem making them up obligatory, even though it differs from what we believe and rule. Since you have chosen to emulate this view, you should increase your acts of seeking forgiveness (istighfar) and performing supererogatory prayers (nawafil). As for fasting, you must make up the missed days. You should estimate, to the best of your judgment and strong presumption, the number of days you broke your fast and make them all up. There is no harm in using allusions (ma'areedh) to avoid embarrassment when stating that you are fasting to make up for missed days.