So, I happened on this thread late, and am currently working my way through the process. Here's what I've done so far:
Using an actual Android device (LG GPad 5), running Android 9 (developer tools already enabled), I installed 4.16.... of Kindle for Android, then signed into the app (so it knew who I was and what books I had available). I then downloaded ("Download To Home") the books that I wanted to import into Calibre.
Next, using a laptop computer running Ubuntu 22.04, I installed the ADB tools (using apt install adb). All good there.
Used ADB to copy across the key backup (backup.ab), which resulted in a file about 17M in size.
Downloaded DeDRM from the NoDRM/DeDRM Github page, and installed it into Calibre. Added the backup.ab key successfully (I believe - at least, it prompted me for a name for the key, and it shows up when I try to configure the DeDRM plugin).
Next, I copied from the Android device to the computer the PRC files that I had downloaded using K4A. Then, as I understand things, I use Add Books to add them to the Calibre library, at which time they should be automagically stripped of DRM, so that they can be used as a backup in case something bad ever happens to Amazon
This is where my problem begins - when I import the PRC files into Calibre, they go through the import process, and all the metadata seems to come through no problem, and they show up in the library list. But when I go to view one (using the ebook viewer built into Calibre), it says the file still has DRM and can't be viewed.
I can expand on any of the steps I took, if needed, or supply more information about devices/software used.
I am reading through the thread, but so far I'm only to page 15 out of about 45 pages. But so far, I haven't seen a report of this problem occurring.
So, please, somebody tell me that I'm stupid and where I made my wrong turn
