I had quite good drops from Dynamis. Maybe OP just having a really bad RNG there. lol
Also, you might need to proc those NMs before killing them to raise the chances obtaining rares.
BTW, instead of cramming all your relic drops onto your mog storage, use Porter Moogle to store them (which also stores alot of special/event gear/items). Each cities has one of these npcs. Just be careful not to lose your storage slips, or all your hard-earning drops will lost as well. lol
There's also tons of item management knowledge players requires to know if they want to survive the hoarding onslaught by learning from here.