Bootcamp Assistant runs... then nothing?

Hi folks,

I have a 2015 iMac 5K with a 2TB internal drive (about a1/4 full) and 24GB RAM. I'm trying to install Windows on a Bootcamp partition. I am running the most up-to-date version of Big Sur and have the most recent Windows 10 ISO on my desktop.

When I run Bootcamp assistant everything seems fine. I create a ~250GB partition and the select the Windows ISO. The assistant runs, takes 5-10 minutes through all the steps on the progress bar, then the iMac appears to restart as it should. I.e., screen goes black and I hear the startup chime.

Then... nothing. The screen stays black. I think the Mac might still be powered on somehow (if I connect a USB hub, the lights on the hub come on), but the screen isn't the glowing kind of black that would indicate that. The only think I can do is press and hold the power button and manually restart. Upon doing so, I can see that the BOOTCAMP partition seems to have been created, but it contains zero items.

Things I've tried:

• I've erased the Bootcamp partition and redone the process several times.
• I thought maybe I was being impatient and the computer was actually doing something while the screen was black, so let it go for about 12 hours on one try before I gave up.
• I tried re-downloading the Windows ISO in case I had a corrupted download.
• I tried connecting an old Dell monitor in case Windows was starting but couldn't address the 5k iMac display for some reason.
• I tried holding option down when restarting. Did this multiple times and usually only the Mac volume was available for starting. However, one time there was an option to boot from the Windows partition. When I selected that, though, it just went to the black screen again.

Any thoughts on this?