Luckily you already have Swiftcast which is the absolute best cross class ability for mages, so if you feel like changing jobs you won't need to come back to BLM for abilities.

For what it is worth, BLM gets a bit faster paced as it levels up. Fire 3 and Ice 3 both let you swap between max fire and ice stacks immediately, and Firestarter and Thundercloud get you some luck-based triggers to pump out some extra damage.

You should under no circumstances make a new character just to change jobs, IMO. Not only will you have to unlock a bunch of stuff all over again, changing to a new class will give you a permanent 50% XP bonus until it becomes your highest level battle class and it really speeds things up.