I am a pc player, and I can tell you... the 30 seconds cooldown is just for show. There is another cooldown that lets you get x amount of refinement points or x amount of refinement material within a certain time. So after a while, you are not getting any refinement materials from the dragon's hoard.
The dungeons should be playable according to the suggested minimum item level. If a proper group with item level 1600 attempts to do an 1600 dungeon it should be hard for them BUT doable in my opinion.
I have had the black opal drop twice for me this week, that is not the problem for me at all. It is just that I feel like I hit a wall with getting any refinement material drops.
Maybe a good question to pop in here as well: can DF proc the bonus of the lostmauth set multiple times in the 3rd attack? I believed it does, which makes it more interesting for me to use DF.