I was in a similar situation as cowchair with my Kobo Libre. I flashed the revision E image onto a new microSD card using dd and gparted to expand the FAT32 partition.
It worked, but unfortunately I'm unable to update the software anymore. It checks for updates, downloads the update, but after restarting its stuck on the same version. I got past the setup stage using these instructions:
https://yingtongli.me/blog/2018/07/30/kobo-rego.html and tried manually updating by extracting the update into the .kobo folder, but still no luck.
It's stuck on software version 4.32.19501. Also has a blank serial number.
As of now the eReader is still usable with Calibre, but unfortunately I can't download from my local library with overdrive, as syncing from overdrive also triggers a software update check.
Did anyone have this same problem?