The major problem you may be feeling at the moment is that, while Astrologian is, indeed, a caster, they are not a magic DPS job. They're DPS is going to be low and slow while leveling because, well, healers are not on par with the top caster DPS, and were never intended to be.

Black Mage is always casting, not having to worry so much about casting resources. It sacrifices mobility for this. That said, you can slip away from AoEs after initial casting with a little practice. Take the chocobo out for heals during leveling and fear hardly anything.

Summoner has a play style through level 30 that is basic (as are all of the original ARR classes). It has a straightforward DOTS+rotation that only really begins to be effective around level 52. It is also the magic DPS that will apparently change the most in the next expansion, so you might want to try it out again after that.

Red Mage builds up resources via ranged magic, and burns those resource via melee attacks. If you've tried the playstyle, just realize that it was designed for Stormbringers, and the playstyle starts to change up a bit between 60 and 70.

Blue Mage isn't really a job. It can be quite fun, but it is not viable for MSQ, so treat it as a mini-game, to be played when you're bored of other things.