In my experience, out of 20 kills on Provenance Watcher I've seen more Meteor and Arise then I have actual gear. Why not take one of the material slots and change it to one random Head, Hands, Legs, or Feet abj. This would allow people to get some reward for clearing the final boss, but at same time there is 20 different abjs that can drop in that slot.

The trove would look like this:

Random gear, body abj, scroll, or mat if no gear drops in slot 1.
Random head, hands, legs, or feet abj in slot 2.
Random material in slot 3.

This way you are getting somewhat rewarded, but still not getting everything right away. Getting 2 Red rocks and a Gold ore is pretty sad for the final boss. Also one more slot should be opened up from the direct drop pool. Maybe for a random Body abj.

So the pool would look like this:

Slot 1: Sword, Staff, Helm, Shield, Tes Body, Hat, or nothing.
Slot 2: F:body, S:body, C:body, T:body, L:body, or nothing.
Slot 3: Meteor, Arise, or nothing.

For a final boss of an event, Provenance Watcher has a pretty crappy drop rate.