If you have attuned to the Aetheryte in Zahar'ak a second time, you would have finished the quest from Thancred. It just opens up Bowl of Embers (Hard). You need to complete that, once, in order to get the next quest.

All three hard-mode quest starts are in the Waking Sands. They have you travel to re-attune to the Aetheryte and that finishes the quest. Completing the Hard-mode trial then makes the next quest eligible.

It's not "get quest, do trial, complete quest". It's "get quest, attune to Aetheryte, complete quest" then "do trial".