I’m using Calibre Web and it syncs with Kobo just fine. It’s a minor change to the config and then you can sync with your own library.

Self hosted in Docker, I browse to the book I want, add it to a ‘shelf’ that is synchronised. then on the Kobo I press sync. It downloads.

I used a guide that was something like this:

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

i kind of use witbound instead of whispersync. it was one time purchase and now i get sync my m4b and epubs together and not be locked on whispersync.

i kind of use the witbound on the app store, my own files epub and m4b instead of being locked using whispersync.