Does one sin if he does not make up missed prayers after learning of the obligation to do so, and is that considered following one’s whims?

There is no harm in adopting the view of not making up (the fast), and this is not following whims but rather a dispensation due to necessity, the ease of the religion, and the removal of hardship. This is a valid opinion supported by evidence. One must completely turn away from obsessive thoughts and pay no attention to them, as they lead to extremism, undue burden, and hardship.