finished a arrb but can't get 'before the dawn quest' where is it?

[Moomba33]

There's about 100 quests after The Ultimate Weapon to reach Before the Dawn so you have a ways to go.
You'll get more credits after Before the Dawn as it's the end of the ARR patch quests leading into Heavensward.

While it's true you can't get your 52 class skill until you reach the expansion you also won't unlock any content that has you above 50 until the expansion anyways.
(Palace of the Dead will let you go to 60 before then, but it also lets you use job abilities you haven't done the quests for so it's alright).