I've done the full factory reset a couple of times now:
https://help.kobo.com/hc/en-us/articles/360017765853
Basically, power off the unit, then, while holding one of the page turn button, press and hold the power button until it resets.
It definitely resets the firmware version back to something a year or so old. You'll need to sync and update afterward and then put everything back on.