Your Whitemage tops on the Conjurer. If you unlocked it you start from level 30 the xp you get will add to the baseclass conjurer and your whitemage will level along and you get all class skills for the job as well available.
Your job has a job quest where you get nice additional skills all 5 levels so its 30, 35, 40, 45 (first bits of AF gear ) and level 50.
Once completed the level 50 quest you can start the relic.

Also you will want for endgame some certain crosskills (leaving to you if you level along the way or once you hit 50) that means to bring the arcanist to level 34 for eye for an eye, virus / thaumaturge to level 26 for swiftcast and surecast.

Besides that you generally dont want to ever switch back to class unless the level 30 conjurer quest where you get the white unicorn and later for the fun