Mac – Boot Camp ‘saving support software’ holds indefinitely

So, there's quite a few other “Boot Camp Assistant's handling of the ‘Windows support software’ is terrible” questions; but as far as I can tell, none of them directly relate to my problem:

On a machine on which Boot Camp Assistant doesn't give the option to use external install media¹, I get stuck at the “Saving Windows support software …” step, when the progress-bar is at 100%:

Screenshot of Boot Camp stuck http://ell.io/i1l8hv+

I've left it sitting at that screen, this time around, for around 24 hours. I do have a rather large disk (2TB OWC Aura SSD), so I decided to give it some time … but it's pretty clear at this point that nothing is happening. (I'm also afraid to stop it; because last time I gave up on it, it took a solid hour to ‘remove created partitions.’ Took nearly as long to un-do, as I'd given it to do. :P)

To be clear, the download phase completed; it's stuck at the ‘Saving’ phase. Unlike old-school Boot Camp, I can't just uncheck the option and download the drivers manually; it forces me to go through this step.

So, tl;dr: Is there any way to skip the ‘downloading and saving drivers’ steps, or use pre-existing install media, or otherwise get Boot Camp working on a MacPro6,1, or "2013 Mac Pro"?

Mac-side: macOS Sierra 10.12
Windows-side: Windows 10 Pro 10.0.14393

1. Older Boot Camp, and current Boot Camp on some machines, gives you a list of checkboxes to decide what you want it to do. Unfortunately, current Boot Camp on the 2013 Mac Pro does not do this.

“If the Select Tasks step doesn’t appear, you don’t need a USB flash drive or hard drive to install Windows. Follow the instructions in Install Windows on your Mac without using a USB hard drive or flash drive.”

2. I've also taken some additional information — current disk partitions, process-sample for Boot Camp Assistant (spoiler: it's sitting around doing nothing.), and etc: https://gist.github.com/ELLIOTTCABLE/5f0c13cbe38a7ab952b82ec11ae4ed2e