I couldn't find information on google whether you can assign the rider as a diff and merge tool in GitExtensions. There is no such option in GitExtensions by default
On GitHub Gist, there is only one mention of setting up rider as a global diffftool. However, even if the path to rider64.exe is specified correctly, and GitExtensions itself sees Rider as a diffftool, nothing happens when you try to run the application. Regardless of whether the IDE is open or not.