I second AttacKat's answer. Any reasonable profit (and XP) in crafting comes from HQ items, and for that, you'll have to rely heavily on cross-class skills, so it's difficult to succeed at leveling only one job. Definitely level everything to 15. After that, you can decide if you want to level classes to 50 one at a time, or level everything together (so you can make your own cross-class materials or share gear).

End-game, multi-star HQ crafting requires multiple level 50 classes for the skills necessary to be competitive. I recommend taking a peak at one of the many ?-star rotation guides and checking the required skills listed. But in the meantime, the lvl 15 class abilities will help tremendously while you begin your long and tedious journey.

Also, if you have no shortage of leve allowances, I recommend the "delivery" leve quests that send you to other locations. These offer less XP per leve allowance than repeatables, but highest XP per item turn-in.

Happy crafting!