Just curious, for this application where I don't care about how many times the battery has been cycled...why wouldn't you just buy the Fire HD 10 refurbs for $80?
I would totally go for the 10 if it wasn't because that one still uses the old Fire OS 5 which is still based on the now ancient Android 4.4... I believe the new 10 with Fire OS 6 (Android 7.1) will be released later this year...
I'm undecided between going for the 8 at this price or waiting for the new 10. A good thing about the 8 is that the XDA gods manage to figure out a way to put LineageOS on this one and it's almost stable, still a couple of wrinkles to iron but they are actively working on it...