RDM Reforged Artifact Gear and Stymie.

[Protey]

with the upgrade to relic body enhancing skill, it will be possible to get 500 enhancing without having to swap in that enhancing skill dagger and still max out on enhancing duration. Here's how:
404 base
16 merits
8 augmented zenith crown +1
7 enhancing or colossus torque
3 augmenting earring
19 Vitivation Tabard +1
6 estoqueur's cape
5 olympus sash
17 atrophy tights +1
15 estoqueur's houseaux +2
total: 500
can also swap in an augmented aptus earring for +2 (obtained from chocobo key from Battaru Royale mission from Shantotto Ascension expansion) if want to use the enhancing +3 waist instead of the +5 or can't quite get that +8 augment on head.

edit: just saw the Umuthi Hat with 13 enhancing skill. 500 is really easy now. don't even need to bother with the earrings.

[Raydeus]

with the upgrade to relic body enhancing skill, it will be possible to get 500 enhancing without having to swap in that enhancing skill dagger and still max out on enhancing duration.

Edit > I know you wanted max duration but it just isn't worth the extra pieces of gear. Because as many have said before me that extra duration should've been put on the tights, not the freaking gloves.