I've been looking at using an older Kindle of mine, a 4.1.3 device, to try to force it to d/l anything rather than KFX-ZIP files...
But (i) having deregistered it I can't reregister it (ii) after a bit it pops up a message from Amazon saying words to the effect that as it is so old it won't connect to the Kindle Store anymore - great!.
Also, the time is an hour out (DST) which may be causing issues but I can't see how to re sync it.
Some good Redit threads on this - such as -
https://www.reddit.com/r/kindle/comments/tcps95/unable_to_register_amazon_account_on_kindle_4th/?utm_name=androidcss
So it looks like Amazon have closed off another backup possibility. Bah.