There is an API that you can call to sync Kobo e-readers with calibre.

https://jccpalmer.com/posts/setting-up-kobo-sync-with-calibre-web/

It just depends on if this would work with your current thoughts on how your app works.

I would be interested in something that can sync with Calibre-Web

Kinda cool how the configs work, but it feels like you need to rely a little too much on having premade configs available for the things you want to do.

https://arstechnica.com/gadgets/2025/03/roku-says-unpopular-autoplay-ads-are-just-a-test/

https://arstechnica.com/gadgets/2025/03/i-threw-away-audibles-app-and-now-i-self-host-my-audiobooks/

Check out this arstechnica article on AudioBookshelf. Should cover most of what you need to get started.