Dark Sun was a great setting. It is very much a product of the 90's, warts and all. It could stand to be updated, not watered down, but updated to be as groundbreaking as the original. Muls, like 1E half-orcs, need a rework. In a world of ecological apocalypse and magical tampering of evolution, why would Muls necessarily be 'bred' for slavery? Muls are just another way, like psionic ability, the natural world is making stronger candidates for survival.
Chattel Slavery doesn't need to be ever present. The Sorcerer Kings are in complete control. The templars, merchants, and nobles owe their position to them. The rest of the population is similar servitude without the need for echoes of history. Food, water, and safety are all limited resources. Control is easy for those with power. Slavery takes many forms after all.
I'd do a complete reboot. Not just a calendar advance. Set 5E Dark Sun after the appearance of the first Avangion. The Veiled Alliance becomes a crusade of preservers, a scourge to Freed City-States and those under the yoke of Sorcerer Kings. The Dragon still exists. How will the party navigate this chaotic land?