Why are you posting what the Round Earth Theory claims is an illusion to show that there is no illusion occuring?
Why are you appealing to well known refraction, which easily explains it for a RE, while avoiding providing a FE explanation?
Note: refraction is not an illusion.
Also note: refraction is well understood.
With a RE we have a very small amount of refraction, less than 1 degree.
For a FE you need massive amounts of refraction which varies dramatically based upon location and time.
At 10 000 km away, the 5000 km high sun should appear at an angle of roughly 26.5 degrees.
That means you need 26.5 degrees of refraction, and more importantly, in the opposite direction.
What causes this opposite direction of refraction?
We know that as altitude increases, density decreases. If it didn't the dense air would fall.
We also know that for air, refractive index is related to density such that more dense air has a higher refractive index.
This means as altitude increases, refractive index decreases.
So a ray of light going downwards from the sun will go into a denser and denser medium meaning it will go closer to the normal and thus curve down, not up like you need.