If it is helpful to other people here is how I manage my ebook library with flatfiles and heterogeneous devices (Android, macOS, iOS, Kindle, etc)

I have my own eBook library hosted with copyparty. It is a single binary. Runs on plain files. And speaks OPDS for integration with KOreader (Kindle) or Readest (iOS/macOS). https://github.com/9001/copyparty#opds-feeds

It also speaks webdav so I can quickly sync everything via rclone to an SD card on devices like an Android tablet.

I really like the convenience of flat files and multiple protocols.