Apple support, 30 minutes in, resorted to coaching me back to my 2FA acknowledgement email (a day prior) to select the undo options. If you're still within some limited window, you can do that same thing, and that's what I'd recommend.
5000+ people posted here with this same issue - and that's clearly not everyone, just the people that came upon this particular link. Apple clearly should be doing better than this.
And it turns out that, if like me you turned on 2FA only because you wanted to back up your messages, and Apple tries to make you think that's the only way, well, my work IT department tells me it's possible just using iTunes (yeah, I'm no big fan of THAT dinosaur, either, but at least it's not going to lock me out of my iPhone...or so I hope.)