I got a lot from the Twilight Ridge area in Nagrand. I’d semi-afk and get the respawns since they all spawn close together. I’d get the mana thistle and the Dreaming Glory (about 4x more Dreaming Glory). Later when that got more populated I’d got to the Orgrilla area which people don’t like because of the mobs and the cannons.
I’ve also found the mana thistles in the hills by Aldor Rise, Socrethar’s Seat (very camped on my server) in Nagrand, and the Dragonmaw area in SMV. I still fly by these areas and get the thistles sometimes but I haven’t been herbing as much and it is definitely more farmed than when I started, probably because of layers being removed too.