The levelling process for healers is very easy, you can get away with many things on pretty much all three of them. After you get into more serious content you begin to learn more about Cleric timings, overhealing, pre-casting, MP management etc etc.

WHM is great for making dungeon runs faster, but if you're using Assize and Shroud appropriately, as you say, then there's not much else you can do. WHM don't have any other MP regen abilities, aside from using potions. Your two DPS are doing lower than desired DPS, so the pulls take longer, you use more abilities and you lose more MP.

As for rotation, assuming there are multiple targets in one pull, you'll want to Holy -> Aero III while they are stunned, and then basically just Holy spam and weave in an Assize when it's up. Single target wise just get DoTs up (not Aero unless you're moving or you can weave it with Fluid Aura without pushing back) and then fill in with Stone IIIs. The rotation is very simple, but in end game content you won't get too far pushing DPS while also healing full time. There's a reason SCHs have the best sustained DPS, they just have better tools to pull it off.