Can't you get light while unsynced? I would imagine the best way to get light is find a group of people that want to unsync Final Coil turns, like turn 12? Level 60's can thrash that fight in 4-5 mins with like 1-2 Bennu, even if you have a level 50 weapon for light, and I know when light-farming was relevant you always got the most points from the current 24-man raid and the current coil turns. It might take longer to find a group willing to do this, but I imagine it would be well worth it if you can get a decent amount of one-shot runs in.

If not, put up an unsynced Pony Farm PF and use it for light-farming.

Unless you can't acquire light when you are unsynced.. (i have no idea my relic was done in 2.0)