So, i have to finish Story Quest?

I hit 50 and did the quests, and now i'm stuck in The Praetorium cause i can't find a party that can pass through Ultima Weapon... but i really want to do the end-game dungeons and trials.

So, my question is, I'm really obligated to finish the Story Quest if i want to enjoy the other end-game contents?

Don't give upm you're at the final dungeon! And yes, since some other endgame content is story-related (this is a final fantasy after all).