take a second to review (and maybe correct) your post 
Why is BFA the default storyline? BFA was the default when SL was the active expansion. But the pointer for default story line should have been moved to the next expansion, SL, when DF was released. So now I have geared 60 tanks who can’t queue for covenant and world quests that require a dungeon. If they take the BFA option, they get instantly accepted (due to the rarity of tanks) and never get the ten minute timeout that allows one to choose the later expansion.
From what I was told when I started, your first level 60 has to level through Battle for Azeroth, and then your next can use Chromie Time. I don’t know if there are more requirements than that.