I’m rather hesitant on paladins. Their core concept - swearing an oath to a higher ideal, setting aside personal concerns to serve a greater cause - is very much at odds with Dark Sun’s brutal struggle for survival. Warlocks make perfect sense to me, since Sorcerer Kings make perfect Otherwordly Patrons - they’re not gods, but they grant power. I’ve made archetypes fitting the different Sorcerer Kings, or at least my interpretations of them. Half of all the templar’s additional spells match to give them a sense of continuity.
The one thing that would have gotten me interested in 4e was Dark Sun, but it was a bit late to really get in interested. I agree that hitting the archetypes is important, but 4e’s structure is so different that it doesn’t really compare with the example IMHO, but the goal should be the same.
Using the 2e/3.x smash up that 5e is, Warlock is perfect as a class for Templar, with backgrounds filling in the pieces that you’re referring to. Noble or acolyte for a traditional Templar. “Moon Priest” is the Warlock Archetype for Tectuktitlay (I’ve typed that name enough in the past few days that I can remember how to spell it, lol), and I assigned them Moon Beam as a 2nd level expanded spell option with this in mind. Backgrounds cover the rest - a royal defiler would be Wizard with the Defiler archetype and the either Noble or Sage background. Same goal, but a different system or a different route.