I only use flatpak if no deb.
If the author writes "Don't use packages with the Distros, use my script", then that's what I do.
For years I downloaded the Windows version and now run the script for Linux on the Calibre web site.
See what Kovid writes on the download page:
https://calibre-ebook.com/download_linux
Quote:
Please do not use your distribution provided calibre package, as those are often buggy/outdated. Instead use the Binary install described below.
Binary install
calibre has a binary install that includes private versions of all its dependencies. It runs on 64-bit Intel or ARM compatible machines. To install or upgrade, simply copy and paste the following command into a terminal and press Enter:
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