How does one make up for many missed prayers, given one's fear of dying before making them up, and one's current inability to make them up?

A Muslim must make up for missed prayers in a way that does not harm their body or livelihood, for Allah Almighty says: "So fear Allah as much as you are able." So, whoever does what they can and then dies, there is no sin upon them. If there was an excuse for delaying the makeup prayers due to studying, then - Allah willing - there is no sin. It is good to make up two days' worth of prayers with each day [of the present] until one is able to catch up on what was missed when there is ample time.