Yes, of course their perspective was relevant. They saw Him go up from them, which is what He did. Your correlation is invalid however, because He ascended right in front of them. The sun is very far away, and that distance makes the perspective critical, because a different perspective would result in a different description of the events.
How many discussions have you had with someone concerning the "relevancy" of the perspective of those who watched Jesus ascend? I can't remember ever telling someone, "Now when you read about Jesus ascending, pay very close attention to the position of the witnesses and their perspective, because it is very relevant to the event."
Maybe we're talking past each other. Here is my point. Of course Jesus went UPWARDS from the witnesses' perspective. You don't even have to give it a second thought, since nobody thinks the witnesses were standing on a cloud. It doesn't rank very high on the relevancy meter, because it is a no-brainer.
Likewise, for many thousands of years, the lights passing over the earth in the sky wasn't relevant either, because everybody knew that men stood on the earth looking up, and the lights ran circuits over the earth in the sky. You only think their perspective is relevant today because you've been told that the lights are stationary, and the earth is what's moving. Hence my question: Why do you think it
is relevant? The answer is: Because men have told me that the earth rotates and orbits.
But to the men who wrote the words in the Bible, it wasn't relevant at all, since the lights only looked like they moved over us because they DID move over us. Like Jesus ascending, it was a no-brainer.
So here's what you should consider... "all scripture is God-breathed", "scripture cannot be broken", "not one jot or tittle will pass away", etc. So if your argument is that these men were just ignorant goat herders who didn't understand how the world actually worked and were only explaining things from their own perspective, just remember that God's Holy Spirit spoke through those men. So if they were wrong, then God's Spirit was wrong. Also, if these inspired men were wrong about the nature of our world and how it works, what else were they wrong about? If they were only explaining the world from their own perspective, then they were also only explaining God's laws and history and prophecy from their own perspectives too.
I will finish with one more thing, and then we don't have to speak about the shape of the earth anymore if you don't want to. It was God who gave Nebuchadnezzar the dream of the tall tree - and Daniel the interpretation of that dream. On a flat earth, it is at least possible for there to be a tree that can be seen from the entirety of the earth. On a ball, the tree could be a billion miles tall, and it still couldn't be seen from the entirety of the earth. Same goes for Satan tempting Jesus with the kingdoms of the world. On a plain, it is possible for a mountain to be sufficiently tall to see all of the kingdoms of the earth. On a ball, it wouldn't matter how tall the mountain was.
So why would God give Neb a dream and Daniel an interpretation about an event that wasn't even possible? Why would Jesus relate an event to his disciples that wasn't even possible?
Thanks for indulging me.