Since you can export the USB storage now, try this:
At the top level of visible USB storage, enter the filename:
DO_FACTORY_RESTORE
All caps, no extension (be sure of that), no contents.
Eject or Safely remove (whatever your OS calls it)
Remove USB cable
Reboot by holding power switch for 40 seconds or more (ignore any pop-up panels that might appear)
Wait
Ref:
https://www.mobileread.com/forums/sho...78&postcount=1
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There are now two (2) methods to downgrade 6th and 7th generation devices.
You don't dare sign off today, you'll get left behind.