Multiple jobs might work if you feel like you're cheating yourself out of the levelling experience, when the best way to level healers in Stormblood was queueing the same dungeons in a row, ahhhhhhhh. God forbid you play a class that sucks ass before a certain milestone and it gets a crumb of aoe.
Realised I had some sort of starter's ring equipped which was giving me EVEN MORE XP so I've taken that off and things are settling down a bit. Switched to Arcanist and that's a lot more fun than Thaumaturge, I even got to do a few side quests though I'm still levelling up fast.
A lot has changed though. Have they removed the requirement to level a second class as well before you get your proper Job at 30?
Yes, that's been gone for ages, no idea what Arcanist does for SCH and SMN now since you level one you basically get the other for free, only reason to multi level jobs is gir your own fun and flexibility, if you ever log in to your old account 80% of your hotbar will be greyed out