Would you be interested in trying BookFusion?

BookFusion has a Calibre plugin that allows you to easily sync your Calibre eBooks to Android, iOS and Web. The benefit is that all your entire eBook collection will now be at the tip of your finger tips and your bookmarks, highlights, reading progress and eBooks would be synced across all devices.

You can learn more at https://www.bookfusion.com/reading/calibre

PS: Founder of BookFusion and would be happy to hear any feedback you might have.

Calibre is a complex beast that has a bad track record regarding security and will keep using python 2 even after its deprecation.

It does what I need (for instance decrypting Amazon purchases), runs locally and will only process trusted input, not to mention it seems to be gradually migrated to Python 3. I’m not overly worried.

For an upcoming competitor, security is only a feature if it also does everything else I need it to.