... Round Earth model where significant perspective effects and changes cannot occur?
A picture you posted:
Moon to the Southeast: 239,000 miles away.
Sun to the West, over 120° in another direction, 93,000,000 miles away ...390x further.
Close up. Far away. Doesn't matter. Scale that.
Make the moon the size of a dime. (Borrow the one hiding the elephant.)
It's distance from an observer on earth at that scale is 1.14m (3'9")
The distance to the sun at that scale is 442.3m (or a 1/4 of a mile)
The big numbers don't change the relationships. You can use small numbers too. That's what scale models do.
Looking at something 3-4 feet away, and then turning 90° or so and looking at somethig a quarter mile away, and you want to tell us perspective isn't involved?
Now, if you take away any depth perception cues, with the sun at an angular size nearly identical to the moon, your brain (my brain, all our brains) want to imagine a line straight across the sky from that westerly view to the southerly or south-easterly view. But that's wrong. That's the illusion, because you aren't taking depth into account. It's not intuitive to do so. You have to remember that there's a vast difference between the earth-moon distnce and the earth-sun difference.
But that's not the whole story. Throw in different azimuth perspectives and you've got an optical illusion. You are taking in a panoramic image when you look in one direction, note the location of the sun and the turn 90° or more and note the location of the moon. If you take a picture of that, you're projecting such a bowed perspective onto a flat surface. That's what this picture is that you found to kick this off:
If you want to rectify that so it appears as a straight line in 2D , then the straight line of the earth horizon will bow. Something has to become distorted to make that moon/sun path a "straight" line on a picture.
Truth is, it IS a straight line already. It's just that you're brain fools you into thinking it's not. Why? Because you don't perceive the affects of perspective. You don't perceive that the sun is such a grand factor further away than the moon, along a different line of bearing.
I posted this earlier and you either ignored it or didn't understand it.
Here are two 2D projections of the moon-sun in an arrangement that produces the illusion that you say 'round earth theory' has no explanation for.
Both of these are correct. The bottom one is presented with the horizon as a straight line on the 2D projection. But that makes the straight line of the sun to moon look like an arc. It's straight in 3D real life, but in the 2D image (or in our brain when looking at a sky without depth cues), it makes an arc. If we rectify that to make the sun-moon line straight, the way you want to think it should be, the horizon bows into an arc. It IS possible to rectify both to be straight, but something's got to give. You could distort the sky where there is no detail and be happy, but if any straight-line thing were to be projected into that space, IT would appear distorted.
Like it or not, it's an illusion. And it's explicable. Much better, in fact, than the "perspective" explanation you had for how that illusion could be explained in a flat earth model with sun and moon at locations and distance NOT 390x different from each other.