If one has to make up for missed fasts from Ramadan (around 20 or so days), do they have to make it up consecutively or can they fast Mondays and Thursdays only, until they make up the days?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is not compulsory for one to make up the missed fasts consecutively regardless of their number. Rather, it is permissible for him to make them up any time he wants to. It is narrated on the authority of Ibn ‘Umar that the Prophet said about making the missed fasts of Ramadan: “One may fast consecutively and on occasions.” Moreover, Ibn Abbas narrated that the Prophet said: “There is no harm for you to make up the fasting occasionally since Allaah Says (what means): {…and [wants] for you to complete the period.} [Quran 2:185] [Imam Al-Bukhari]

If the month of Sha’ban has come and still one did not make up the fast that he missed last year and the remaining days of Sha’ban are just sufficient to make up the missed fasts, then in this case, he is obliged to fast consecutively because the time becomes very short just enough for him to make them up.

Allah knows best.