> 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.

I dont think there can be intuition for this. How can one even imagine an electron "flying" through space, there is no measurable thing that travels, we can only see the disturbance of the surrounding space caused by it. But what is the it, does it maybe live in other dimension not accessible to us, or is there no it at all ? I gave up as a layman trying to understand any of this. Sure, there is math and you can somehow make some mental models involving 3d graphics and all, but that is not actually what is going on down there.

In a medium, this can create a "phase kickback" which creates a combined wave that appears to travel slower than the original one. The kickback is just the result of multiple EM changes propagating, i.e. the photons interact with the material, re-emitting photons.

3Blue1Brown has a beautiful animation for this phase kickback here: https://youtube.com/shorts/XIW-2ykgVPI?si=PJWiAC2BO7_xP0S6

> Single EM waves propagate with exactly this speed ...

Yes ... and no. Or we should say yes, but not necessarily forward. What about circular? What about ball shaped? What about a vortex? Any valid soliton is a solution and a single wave. Which means this is not just possible in water, but also in the electromagnetic field:

https://www.youtube.com/watch?v=909o_kbCdFgll

Circular (as in 2d circular in 3d space) propagation in EM waves is like 2 waves in exactly the same location and direction, with opposite rotation along the axis. Which could be extremely useful since they propagate like single waves. In other words, there is a non-circular spectrum ... AND a circular spectrum. So, if we modify all radios we have double the spectrum.