Apologies for not being able to reply quicker/individually but thank you all for the advice! I'm actually noticing an improvement on my WHM in lvl 80 dungeons. Only had to use Cures to heal the dps on the second and last bosses in Twinning a couple times. I didn't really feel overly stressed about the tank's hp.

So I guess my problem (for WHM at least) was proper skill usage/timing and gear. Got some melds to help until I can get my Eden gear (I didn't know melds were so important lol). I'm using regen and the afflatus skills more now - in particular afflatus rapture, I didn't know the radius was so large I think I relied too much on Tetra and Benison and forgot all about my lilies before. Being able to weave in oGCDs between heals using lilies is making a big difference.

As for SCH, I'm going to try it in a few 80 dungeons and see if I'm still trash, lol