I'm not a physicist, but AFAIU the speed of travel/causality for light is only the maximum speed of causality because photons have a mass of literally zero. If

I'm not a physicist, but AFAIU the speed of travel/causality for light is only the maximum speed of causality because photons have a mass of literally zero. If photons had non-zero mass they could only travel slower than the maximum speed of causality (which would probably be called speed of gravity rather than speed of light, in this alternative universe).