The actual velocity of one particular guage boson in free space is a happy accident and a convenient way to measure c, but “c” is much deeper than that. It’s a structural constant of spacetime, and whatever its value happens to be, spacetime will behave as the appropriate field equations require with that value of “c” subsituted in for the lorentz tranformations etc. What actual photons do is largely irrelevant since einstein’s field equations would still be true if there was no charge and hence no EM fields to give rise to photons in the first place.
Well said. Measuring the actual speed of light to “prove Einstein wrong” is like claiming a new paint job on your car invalidates Newton’s Laws of Motion.