By all means level multiple jobs, cross class abilities are important and it's fun mixing it up. I would recommend leveling both a tank and healer, first of all you'll better understand the roles your teammates play even if you just DD most of the time, but also you'll have the flexibility for duties, long queues for DD then go as your tank or healer, need to complete a certain duty or want to help friends but all DD party slots are taken, then you have the flexibility to switch roles.

Try them all out, level them for the cross class abilities and see which one you enjoy and are most effective as, but realistically you'll only have one "main" when you reach endgame, as said before the gear investment and also you don't want to become a Jack of all trades but a master of none, as you may struggle at end game.

Hope this helps

PS. End game is the best part of this game imo so getting there asap was my objective when I started and what I'd recommend.