So here's a little stupid trick I just figured out, which may not be useful going forward, but still saved my bacon (or at least a half hour of driving and $40):
Breaking out of setup flow into normal System Preferences during setup
Situation: in charge of setting up a factory reset/reinstalled used iMac at a client's, they said they had everything including of course the basics of... mouse and keyboard. All I've got with me are phone/MBP. But it turns out the used keyboard was a Logitech wireless one, and like apparently all such keyboards it has zero normal USB functionality. The USB port is only for charging (to my utter fury, how I loathe wireless only keyboards). So system boots into startup flow, I can mouse through it for a bit including skipping connecting it to the network, but then comes the time to make the initial user account and I'm stuck. No dialog for connecting a bluetooth keyboard, and I've got no other keyboard! I'm tooling around when I remembered an old trick for naive kiosks, the good fun of trying to get out of the locked flow they want you in and back out into the full OS. Sure enough once I get to the account page, the field to enter a name is a normal OS native entry widget. That means full right click options are there, including hoorah! Text Substitutions.
Which some Apple dev wanted a way for people to get to directly. So right-click: Substitutions -> Show Substitutions brings up a dialog box, and one of the buttons below is Text Preferences... which sure enough opens up System Preferences to text prefs, and then in turn every other system preference. Then I can connect the bluetooth keyboard and I'm set!
Dunno if this one will ever come up again in my life. Obviously one could just try to ensure always having some old cheap keyboard around, certainly have a half dozen at my office. But being able to break out into full system preferences before actually completing system setup might be handy once in awhile.