To add more speculation, the idea from the movie "Tenet" could be a way to effectively surpase C in long distance communications: reversed entropy matter.

You could just send a ship or some kind of vesel with reversed entropy to a star and it will arrive years later, but years sooner that it was launched.

Matter is more or less energy, what if this trick could be done with reversed entropy energy: radio, x-rays, gamma rays, etc.