You can set your Everest cast server to avoid replaying the same artist every "X" amount of tracks. You will find the option on the Everest cast setting page under "Avoid same artist tracks"
The higher you set the value, the better mix you'll have, however it's important to understand you must have a decent amount of tracks in your media and in the playlists for this to work well, you can't set the system to not repeat the same artist every 10 tracks if you only have 9 tracks on your server.