@jimp I appreciate the help.
Had contacted TAC, I think they thought as you did.
I couldn't zero out mmdc0 while booted, and in the haze of the moment, it didn't occur to me to boot off the install media to get to a shell that I could zero it out in (duh!). Ah well.
Before that, I did zero out the larger ssd (btw, it's far faster and more descriptive if you run that dd command with "bs=1024k status=progress") without any improvement, so it was definitely something in the mmdc0, because everything went smoothly after zero'ing out that drive.
Thanks!