You should focus one job to 50, as when you get to 50 you get a 50% exp increase. There is a lot more to do at level 50.
Fates IMO are only really worth it once you have the 50% bonus~ Dungeons are the best bet, and if you are waiting for queues, do fates while you wait to make the best use of your time.
Eat food for the 3% well fed bonus and if you are in an FC, see if they wouldn't mind turning on the company action that increases EXP earned via battle.
