You do realize that in order for this to be a perspective effect the sun would need to change its distance to you significantly throughout the day. The 93,000,000 mile sun in the Round Earth model does not change distance to you appreciably throughout the day, and so a comparisons in this thread to one end of the hallway seeming lower than the ceiling above you is inappropriate.
The T-Rex illusion, for example, would not happen if that T-Rex were 93,000,000 miles away in that scene -- you would need to travel for millions of miles around the T-Rex to recreate it (assuming you could see it).
The sun is so far away from you in the Round Earth model that there aren't really any "perspective effects" that it could apply to it. The excuse of a perspective effect for this phenomena under RET is an absurdity, and is in no possible way a mechanism for this "celestial sphere".