I also don't remember hearing anywhere that Shadowbringers would end the "Zodiark arc".

It's going to involve the final Calamity (or thwarting it), but what lies beyond that? Is there something else at the heart of the universe that we need to resolve before this story comes to a close?

From memory, they had said a long time ago that they had the story planned out up to 6.0, so it would make sense that it's all one storyline rather than a grand multi-part story and one stand-alone part at the end of it.