It seems that some optional classes in Xenoblade Chronicles 3 are obtained with the completion of quest lines, or by allies that can join your party.
I am trying to read as little as possible before jumping into this game and during gameplay to avoid spoilers, but also do not want to miss any of these additional classes along the way.
Are any of the optional classes missable? Or, more specifically, are there any that can no longer be obtained after the main story is complete?