Well, the bad news is that I ran README-18.104.22.168 and everything downloaded and wrote - but now it's bricked.
Doesn't initialize networking enough to request DHCP address.
And my usb console cable doesn't seem to fit - so only if I hold it just right does the USB device show up in the System Profiler of my MacBookPro, and of the two serial drivers, none seem to recognize the device. So, I'm going to bail for now. (Have another cable and system elsewhere to try later.)
Thanks for your notes, btw!