> changes in the EM field propagate always with the speed of light in a vacuum, i.e., c (also known as the speed of causality)

Speed of causality is a way more intuitive term.

Massless particles travel at the speed of causation in a vacuum. (Usually. Someone else brought up solitons.) Not necessarily in a medium. Trying to work backwards from speed of light to gravity propagating is tortured; understanding that gravity can't cause an effect faster than causality itself is more direct.