This is how it works:
1. Side Quest available?
Yes
=>Do Sidequest
No
2.Main Quest available?
Yes
=>Do Main Quest
No
3. Grind until you can accept new quests
Grind exp is always decent.
You can avoid all side quests and just grind.
But you need the main quests to unlock new areas.
And the exp cards from the quests are faster than the grind.
If you just do quests and never grind, you will be forced to grind to get to the new quests at certain points in the game.
TL;DR:
Grind+Questing = Fastest lvling process
Only Grind = can’t unlock new areas to grind at
Only Quests = you will run out of quests



find a party so you won’t get bored.