It worked! I ran the factory reset as described, flashed it with the listed modduo-v1.13.4.3293.tar factory reset image, and the upgrade was successful!
Note: The upgrade took quite a long time (maybe 6-7 minutes with the red LEDs on after I unplugged the USB) - longer than I would have guessed, but it worked just fine.
And then I’ve updated successfully to v1.13.5 using the normal upgrade method.
Thanks so much! I’m going to have so much fun with this over the holidays…