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.
I have no relation to BookOrbit except I'm a very happy user.
(I have my Calibre library in iCloud Drive, but if I wasn't inside the Apple Ecosystem I'd use R2)