@AzureFlare
Notice that I didn't say nocturnal Astro. I don't see N. AST as an off-healer version of D. AST because your toolkit is still there and you can still main heal some fights in Nocturnal, even with a SCH in the group (A2S, for example). In my opinion, it's more a matter of play style than a matter of duty requirements.
Also, WHM/N. AST is a good combo for Thordan. The constant moving around the arena, repositioning the boss and constant AoE damage allow for N. AST to shine a bit in areas a SCH can't. Also, a lot of the SCH's DPS is lost when Shadow Flare becomes hard to use efficiently; if we're just throwing some DoTs and adding some burst spells when we see the boss doing his basic Dragon Eye + Knights of the Round rotation in the beginning of each fight, AST's combined potency can be higher than SCH's. When I cleared Thordan for the first time, I was playing D. AST with a SCH; I solo healed the first phase while adding some DoTs here and there and he DPSed the whole time, he also DPSed a little bit during the two knights part while I solo healed, and on the phases after Ultimate End, we DPSed the same amount of time by adding our full DoT rotation and some burst spells at the star of each phase. At the end of the fight, he parsed something around 360 and I parsed 280. Only 80 more, and he DPSed way more than I did. The second time I cleared it, I beat the SCH in DPS, and I had to solo heal the first phase with almost no DPS at all, only adding DoTs at the start of each phase. We're not as far behind as some people want us to be.