Windows 10 setup stuck at "Just a Moment"

A partial answer:

I found a post on a Microsoft support website supporting my speculation that power-cycling could work. So, following it, here's what I did exactly:

1. Powered the machine down with a long press on the power button.
2. Removed the Windows installation media (in my case, the flash drive).
3. Powered the machine back up.
4. Entered the UEFI boot menu.
5. Chose the apparently-newly-installed boot manager on the SSD.

I got that blue-perspective-windows logo for a while, then I arrived again at the "Just a Moment" screen. But a second or so later, the installation continued to checking for updates, or what's new on the network, or something like that, and continued from there on with a couple more settings it wanted to have me make.

I can't guarantee that this will work for everybody in this situation, but - that's my experience. Better than waiting indefinitely.

Also - I have no idea why this happened!