Paltry XP? Unrewarding dungeons? What on earth.............. If you're doing level-appropriate content, I really don't understand how you get that. With dungeons, two, maybe three runs get you to another level (quite often, even one run does the trick). Roulette is also a big boost. As for FATEs and leves, they are obviously meant to be a grind, yet for the small amount of time involved (few minutes, really), they do give a lot. FATEs even more so in a farm party. If anything, it's the Hunting Log that doesn't really contribute to much, I think. The rest of the options though...... You might just be a little burnt out from levelling all those classes? Not sure what you have or don't have unlocked, but there's a dungeon every ~two-three levels, most of which become accessible via story quests. Make sure you do those. And make sure what you're doing for XP is level-appropriate, being synced to lower levels, mainly outside roulettes, might indeed feel slow.

Honestly though, leveling up is extremely easy and fast now. With rest bonus, with food bonus, with everyone and their mothers doing relics/zodiacs... I'd say if you do focus on it, you could get to 50 in about a week. Maybe even less.