When objects move in velocity v < c, their matter states do not change. You remain you, electron remains electron, etc.
When something moves in velocity of c, it will be released photon only. The matter state of released photon is obviously different from you, electron, etc…
The traditional 3D space SR contains a hidden condition: matter state does not change. (Actually, the so called “rest frame” or “moving frame” is attached to substantial object.) That’s why v < c is the premise of traditional 3D space SR.
When people use SR to analyze space – time transformation, etc, concerning released photon, an element denoting matter state should be added. Moreover, anti matter, dark matter, etc, are different matter states…The traditional 3D space SR should be expanded with the element denoting matter states and then it will be sufficient to represent cosmos.
With regard to the question of “light speed in photon frame”, I think that the traditional Lorentz transformation of speed is not applicable anymore because the observer and released photon are in different matter states. People have to find another way to do that.