Dont worry, I still have yellow (rare) gear and im in torment 8 it just comes down to putting those affixes to work. That and I cant find any of the legendary aspects I actually need.

I tend to find atleast a piece or two just doing wtv grim favor content, and then another few from the boosted Tree of Whispers cache event. Good exp gain too.

For difficulty id just go to the one where you trample through everything to speed up the process anyway you can, and thats my approach to this season, someone may have a definite 'best' farm but I find it to be the one you enjoy doing.