All quests minus levequests and dungeons and battlefield instances are a one and done deal so basically your storyline progress is saved and will be there once you ding 18 on MRD to pick up where you left off. Naturally this means for every subsequent class you level there is less and less quest exp to use. It's good though that you figured out your main class relatively early. I leveled both Arcanist and GLD to lvl 30ish before i decided to focus on Arcanist/SMN.

You're main source of exp for your MRD are now FATEs and Hunting Logs and Levequests and Dungeons but since MRD has some AoE abilities FATEs are the way to go for you since you get more exp the more mobs you hit.