Unfortunately the main navigation is designed to be at the top where it was developed (coded) into the theme. The only method is to use the sidebar columns, either the left or right, but as you said, these fall below the main content. You might be able to find a plugin that offers a floating menu that you could use, but other than that, custom modifications might be needed which I might be able to do, but that becomes billable time. Please note that when in a mobile view, the top main menu does become a button at the top. I’m assuming you would like to keep that but not have the menu show in normal view at the top at all?