I really thank you for your hints, guys!!!
I managed to solve the issue with some little workarounds, and above all I was able to use the "Send to Kindle" webservice to sync the epub file with its hyphenation to my Amazon cloud!!!
So, no need for sideloading the book, which would have ended in the know issue of the lacking cover and the unability to sync the book across all my devices.
Here is the recipe:
- Edit your book as usual (I use Sigil 1.9.20 to generate the epub3 final file).
- Import the epub file into Calibre (I'm using Calibre ver. 6.12) and use the Hyphenate This! plugin to generate the hyphenation. Since I'm editing a book written in italian, I installed an italian hyphenation dictionary, which I send with this very message, should someone else like me needs to work with italian (just unzip it and load its .dic file in the Hyphenate This! settings panel). If you work with different languages, you'll need to find a proper .dic file, of course (just search in the LibreOffice extensions repository).
Important: Make sure that the language metadata for the book is set to "Italian" (or to the language you're using) with Calibre's "Edit Metadata" function before using Hyphenate This!.
- Finally, send the epub file generated with Calibre to the Kindle (I configured my device's email in Calibre to quickly send the book, but you can also use your email client to send the file as attachment, or use the Amazon's webservice "Send To Kindle").
And that's all, folks!
Your book will show up in your Kindle after a little crunching by the Amazon servers, and will be synchronized across all your devices with its precious cover and its beloved hyphenation
I'm doing some testing right now with my Kindle (a Paperwhite 11th gen) and it seems to work like a breeze, no issues whatsoever, perfect hyphenation across the entire book, very high satisfaction
Hope this helps