Running missions and getting more rares than anything. Just finished a mission on Grandmaster 2 and looted 3 rare, 1 epic, and 1 uncommon. Ran a stronghold and received 1 masterwork out of the entire thing...which is what I would have received in grandmaster 1. The increased difficulty makes the missions or strongholds much more time consuming for the same or, in my case, less rewards. In my opinion Grandmaster 2 is a waste of time.
You are probably right.
The loot is so unstable. Sometimes you´ll get 4 MW in one SH on GM1. And sometimes 1MW on GM2.
Imho Anthem is not ready as a grinding game at all.
I've run GM2 with some friends and the difficulty curve from GM1 is considerable.
I also see no real improved drop chance with GM and some of us have completed a Tyrant Mine on GM1 with one or no Masterworks.
We've found that the "Luck" inscription seems to be the only way to improve drop quality than anything else. I recommend to stack Luck when available.