Pretty sure only those 10 ship years worth of transmissions would suddenly appear. You're still 20K light years away so the rest of those years' transmissions are still in flight.
Hmm I suppose you're right. I guess you'd see very few transmissions during the journey due to time dilation, and then those 10 years would arrive relatively quickly when slowing/stopping?
I was thinking of this being the first half of the twin paradox, but perhaps for the apparent "time gap" in the spacetime diagram to appear, it's necessary for the traveling twin to turn and head back towards Earth quickly to shift simultaneity plane back in the other direction.
