To be clear, the download phase completed it's stuck at the ‘Saving’ phase. (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. 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've left it sitting at that screen, this time around, for around 24 hours.
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%: 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:
