ICS classes do exist above the 400 level but aren't higher certifications or training per se... they are specialized (as well as some below). Off the top of my head, IS-800 is the base course, with an overview of the NIMS framework, followed by 100.b, an introduction to ICS, 100-level classes for particular situations (FDA, Higher Ed, Health/Hospitals, Federal Workers, Law Enforcement, Public Works, Schools), 200-level courses (application for hospitals and single resource incidents), 701 is MACS, 702 is info for PIOs, and 703 Resource management. As a road EMT, you certainly aren't expected (or obligated) in any way to have all of these...
Hope this helps...