I think you should complete the SB main quest story so you'll be on par for ShB:

It will feel frustrating at the beginning when everybody are doing the new things and you can't. It's actually a nice experience once in a while to try "new" things, maybe have a little party chat trying to understand a boss mechanic, instead of arrive after everyone know everything and you're the only one new. Nothing against it but it can always be like that while a new expansions is a rare event.

You should reach 70 by doing the main story alone so don't bother with repeating duties, just do your more useful dailies.

With preorder you get early access and and exp buff earring. During those few days of early access you can unlock GNB and take it to 70. Earring buff + your FC will have exp buff up, get some cheap food for 3% extra and join all the people doing the same thing, probably doing mass kills of FATEs. Quick and easy. You'll be ready to start ShB with your new GNB 70 in shiny coat.

Until early access, after you have a lv70 job and SB story done you could dwell on the the rest.

It's my subjective opinion obviously but I really don't see why pausing the story now.