I have to say, this game does have more options in leveling than most others, but if you aren't enjoying any of them, then maybe it's not the game for you.

The main quests are designed to be done at every level, so get one more and some more will open up (edit: I realise after you might mean finished entirely, I thought you meant finished up to your lvl. If so go to next part). If you open your recommendations tab, it will suggest some side quests for you.

Aside from that, have you tried your hunting log? Its a good supplement to other methods. To be honest I didnt have to do much fate grinding at all until mid 35ish, hunting log, main quests, and dungeons for quests got me that far as well as jumping in nearby fates when one started nearby