I honestly don't see a reason why the BT are shared throughout all the content. It would make sense to limit it to either 3 quests per tribe (3 for Vanu, 3 for Moogle, 3 for Sahagin, etc...) or limit it to like 6 quests per expansion. Something like that. It would encourage to start the BT if you are a new player and still give you a limit how far you can stress them.

Personally I just did them one by one and are at my last quests for the HW BT (with ARR & SB finished).