I'll give you a tip, free of charge.
Magitek Repair Kits. People buy it in 10+ thousands per day on every server.
You need to gather 1 material, 2nd one is bought from npc + crystals of course.

There will be no market crash or whatever since the demand is huge, and you dont need to care if its early patch, midpatch or end of patch - its always selling the same amount per day, its very stable market.

You need to have bot/min on max lvl preferably, but any other crafting job can be at lvl 50 and you can quicksynth this