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Interesting/novel loading screen for initial load + client-side cache + background cache refresh: That's my go to strat.

And if you have the API endpoints and contracts, you can stub them with something like Mirage.js

Yes, align on what the form the API will take; it doesn't have to be set in stone. Usually, you already know what the stack will look like, so you don't need to wait. Since development cycles differ, you'll always be out of sync. So, I'd make that part of the approach.

I can't recommend mockoon.com for this :) 100%

This. I often get started on a feature and just mock all API responses with demo JSON. Can even simulate network latency or errors. Also comes in quite handy as a concrete starting point for discussion once BE devs get involved. 👍

Or never, how about never?

Definitely need to sketch out an API agreement first then backend and frontend can work semi-independently until the big intergration