But then why don’t we see this same effect when other things move further away in the sky? Why do planes get smaller but the sun gets bigger, for instance? Why do trees and mountains look smaller at the horizon level but not the sun and moon?

The sun is still ultimately much bigger than those objects, and is a light source. I think the general consensus from the FE community is that we're talking around a hundred miles for the diameter of the sun, but don't quote me on that. So in reality, you're still seeing a massive object in the sky.

Frequency is irrelevant. This specific flight is flown occasionally, but there are other flights between Australia and South America that are of similar times and patterns. Additionally, Australia to Africa would have to be much longer than what they are.

I mean this is true about frequency, but as I said, apparently there had been a couple of trips where a FE'er brought a compass onboard the flight, wrote down his readings throughout the duration and then laid them out, and they lined up with the path you were talking about across the flat earth map. I don't understand how that would be possible, just letting you know what I had heard on that topic.

I'd recommend looking into the electrostatic atmosphere, what colors the ionized gasses produce when they're electrified / charged by the sun. You'll find the colors explain why sunrise/sunsets can be as beautiful as they are.