Well, I solved it.
But two things. Normally, with Windows "S Mode", there's a link on the Activation page, "Get out of S Mode", that links you to the MS Store online. That's not there if it's not "Activated". So I was, theoretically, STUCK in "S Mode", if I didn't want to purchase a new license.
I started thinking, well, probably the BIOS is "tattooed" with the license key. So what if I take my new 21H1 USB installer, blow away (re-format) the main Windows OS partition on the eMMC, and install fresh, and it should pick up the key from the BIOS.
Amazingly, that "hail Mary" worked! And even better, I wasn't in "S Mode" once it was installed, just regular Win10 Home.
So, I solved it, but it wasn't without quite a bit of hassle. Most end-users would have been stuck.