[MegDreamsofBooks]

Edit - oops, I should look on the correct PC. I do have the AppData\Local\Amazon folder path, just no "updates" folder in Kindle. Probably because I already declined the update.

My solution has been a very strict connection ban on the Kindle app. I made a firewall rule in the Advanced firewall rule settings that just blocks Kindle for PC from talking to the Internet.

When I need to download a book, I turn off wifi, disable the firewall rule, and launch Kindle for PC with no Internet connection. Once it is fully up, then I re-enable wifi, and click the Sync button in K4PC. Download my books, quit K4PC, and re-enable the blocking firewall rule so it can't do sneaky things in the background.

Because it's open for such a short time, and can't "phone home" to check in, this seems to have solved the automatic update problem that I had when I first installed it. But the real solution for me is that I mostly buy my ebooks elsewhere, and only use K4PC on the rare occasions I can't get a book anywhere else and want to buy it (and download a copy for backup) instead of just read it via Kindle Unlimited on my tablet.