Converting KFX copied from Android Kindle Reader

I need to de-DRM and convert a book I bought because I can't read it on a hand-held device at my age. I have installed the DeDRM, KFX Input, Gather KFX-ZIP, Package KFX, and KFX metadata reader plugins into Calibre 2.55.0 on Linux.

But all the instructions I have found keep referring to a "KFZ-ZIP" file. I don't have one: on the Android device, the files are in a directory in
storage/emulated/0/Android/data/com.amazon.kindle/files which I have uploaded to my desktop system, and that directory contains

a DRM voucher .ast file
six .kfx files
autocompletecache.ser
EndActions.data.<dirname>.asc

If I use Add Books and select all files in the directory, Calibre says it is adding nine books, which it does (!) treating each file as a separate book, with one of the .kfx files correctly identified with the title and cover image, but opening it simply opens the disk directory where Calibre has imported it, instead of opening a reader window. Converting it throws the error that the component .kfx files must be in a KFX-ZIP file, but it's not clear what else that should contain, or what its structure should be. There's also nothing to say whether or not the plugins I installed were actually used.#

I'm probably Doing This Wrong™ but so far I haven't found any information about how to do it right...