I don’t have all the answers. These are some of my thoughts.
Innocent children sometimes suffer much, while guilty adults sometimes suffer little. So the suffering that one experiences is not directly and proportionately related to how much one sins.
However, generally speaking, if people in the world sinned less, then the world would have less suffering. Think of all the suffering caused directly by some persons’ sins. Without grave sin, there would not be war, or serious crimes. Without grave sin, there would be no murder, aggravated assault (causing bodily injury), rape, abuse of children. Without grave sin, there would be no racism, slavery, and hatred. The amount of suffering that would disappear from the world if there were no grave sins is immense.
Other sufferings are indirectly related to sins, as when people are harmed by another persons’ selfishness — but by means of the disorder caused when many persons behave in a self-centered manner. World hunger is not cause deliberately by anyone, but is a result of having so much greed and callousness toward others in the world. The same for poverty and similar sufferings.
Disease is not caused directly or indirectly by sin. However, if people did not sin gravely, they would, I believe, end up being much more loving and more compassionate toward others than is the case today. And this would result in better healthcare and therefore less suffering for those who have a medical condition.
God permits us the use of free will, and this implies that people can sin. Then sin is the cause of most of the sufferings in the world. The sufferings that would still be left, if people simply did not sin seriously, would be lessened in effect by the help of selfless loving persons, freed from sin.
Could God not do more to lessen our sufferings, despite our sins? He already does do this to some extent. We cannot know the many grave sufferings that God averted by His providence and grace. And we cannot know how much harm God averted also in each of our lives.
Why doesn’t God do more to lessen the sufferings of the world? If people would pray more, God would do more. We have the responsibility to lessen the sufferings of others by prayer. God has given us a means, but few persons use that means to its fullest effect.
I don’t know all the answers to these questions on suffering. These are simply some of my thoughts on this topic.