Concentrate on one Job first or diversify?

[Anova]

It's really up to you which classes you want to level and when.

That said, I would recommend you start your gathering and crafting classes together, and after around lv.20, when you get your chocobo. The ability to quickly travel to gathering nodes is just too useful to pass up and it will save you a ton of money to gather mats yourself. You should also have access to retainers at that point, giving you extra storage space to store your crafting mats and allowing you to regain that money by selling on the marketboard or using ventures to get more mats.

You can also level your gathering/crafting classes at the same time as you level other dps classes. Healers and Tanks generally have very low queue times in DF, so there's less time to gather or craft while you're waiting. DPS on the other hand, may have up to 30 minute queue times, which makes gather/craft ideal for those times you are just waiting to enter a dungeon.

Also, if you want to level a cross-class, I would recommend THM to 26 first as a healer. Swiftcast is one of the most impactful spells you can get to prevent a wipe. If the tank dies, you can bring them back up in 2 seconds instead of 12 (including animation time for ress). That could be the difference between no dead dps and everybody dead.