I managed to get the video to sync, and downloaded the project, to my Android phone, Samsung S9 - so thought I would render from there... That looked like it was rendering fantastically. Though can only export to Youtube not download to the phone. No matter, I tried putting it up into my Youtube account.
I thought I'd found an (annoying but functional) workaround...
But that has a DIFFERENT issue (frame rates, audio rates, or maybe some kind of settings cross compatibility from desktop to phone, or something?!?)
See the mangled result here (this plays just fine, on the phone, and on Rush on my laptop, pre exporting):