Yes, the game is unfortunately rather short on quests, particularly given its focus on leveling multiple classes with each character. Your hunting log plus a few FATEs or Levequests should get you to 15 readily enough, though, and after that, dungeons provide a great source of XP. So the lack of quests is more of a lack of stories rather than a lack of XP sources while leveling later classes.

As to Black Mage needing Archer, it doesn't really matter that they have a different main stat. Archer provides the Raging Strikes ability to all the ranged damage dealers, whether DoW or DoM, and that 20% damage boost is really useful. So is the aggro reduction of Quelling Strikes if you get Archer the rest of the way to level 34 (though only the first 15 levels are required). The lore explanation of why the two would be related is that learning archery gives you a better handle on the aiming of ranged attacks, which you can then apply to making your ranged spells more effective.