Do the Bosses in that area reset or respawn per day Dragon, Cultist Leader, End Boss?

If not where is the best area to farm for gear drops?

None of those bosses respawn or reset.

The place to get the best gear is Umbra exchange. The best ways to get enchanted items is rescuing captives and asking for material reward, or opening treasure chests or hidden sections.

The Cultists' Hideout and Marsh are probably the best places for farming gear, but are each are quite difficult. Though if you are after some specific types of gear(especially for mid-range gear), there can be some better locations.