My current setup is to host the audiobooks in a Plex library and use a third party app like Prologue [1] on iOS or Chronicle [2] on Android which both work really well for offline listening and progress tracking.
[1] https://apps.apple.com/us/app/prologue/id1459223267 [2] https://play.google.com/store/apps/details?id=io.github.matt...