Yeah the number doesn't describe how high your level cap is, it describes how high the game will handicap your EXP growth as you level up and reclass.
(Without reclassing, your max level is 40 for most units based on their default class, assuming they have an advanced class for that class path. 20/20. 30 for classes like villager that don't promote.)
It sounds to me like that as long as you plan to continue reclassing, there's litttle point in going past level 15 in any class (base or advanced).
Only once you have the skills you need, or are on your last class path, should you level up to 20 before promoting.