Buggrit, still borked.
I did another factory reset, selected my language, plugged in and updated via Kobo Desktop when prompted, waited till I could safely eject, ejected, waited a bit longer to be sure, unplugged, got the hourglass Updating screen, got the Restarting screen... and then, yet again, just the smiley face and the everlasting march of the squares.
Can't help thinking a return is my next step. I've run out of ideas. Anyone?