I wrote my own KOReader sync server that can be self-hosted using Docker. It is built in .NET and is entirely API compatible with the KOReader progress sync plugin.
Wait...isn't there an official KOReader sync server that you can self-host?
Correct. However, I had one major issue with it. If you expose the server to the public internet, anyone can register an account on your server and use it.
Just don't expose it publicly you say? Well, since most people will use KOReader on their E Ink devices, such as a Kobo or Kindle, there isn't really a way to setup a VPN or a mesh service like Tailscale.
With kosync-dotnet, you can pass an environment variable that will allow you to disable account registration altogether. When someone tries to create an account with this setting enabled, they will always see an "Account registration is disabled" error when trying to register.
Okay, so how do I get it?
It's free, open-source and available over on GitHub.
https://github.com/jberlyn/kosync-dotnet