The standard advice for someone trying not to overlevel the story by too much is to spread the exp around. Your one character can take on every job and class, so when you start to get too much dragoon exp you can always swap to samurai for awhile. This runs into the problem of having to gear multiple classes, and having to level up classes that start at lower levels to catch up with your spot in the story. All of the ARR classes start at level one. Samurai and red mage start at 50, gunbreaker and dancer start at 60(you have to get to 60 to unlock these), and dark knight machinist and astrologian start at 30(the trainers for these 3 are in ishgard, so they remain locked until you complete the story quest that gets you access to that town).