Your question is premised on the fact that a flat earth is a fact.

And maybe it is, and at the same time, it is not. Or, it may be concave, instead of convex.

From the point of view of pure materialism, I could argue about which state: flat, concave or convex is more likely to be factual.

Would that change anything for you? Hardly, I guess, because you have already made up your mind and closed the subject.

To make it simple, try and navigate from Auckland, New Zealand, to Buenaventura in Columbia on a convex earth vs a flat earth. For all intends and purposes, try to find mariners who would choose your flat earth solution. That would be a nice one. The trip would at least be twice if not 3x times as long. But you never hear these people about that.

Now I am not saying there is not more to be said about it. IF our world is holographic, maybe to those convinced the earth is flat, the earth indeed is flat. It does not mean other graphics are less true. It simply means you are living in a different world that reveals itself in its own way.

However, I would be very interested to hear the stories of the flat earth mariners doing this trip I mentioned. Or, if you care to read Laura' s book, the youngest to circumnavigate solo, you may be inspired.

When Eratosthenes calculated the circumference of the earth, there were a couple of assumption connected to it. Although the work itself is tremendously laborious and honorable, the conclusions are not what they seem.

1. It presupposes that the sunlight does not suffer from influence so that the angle of the shade at both Alexandria and Syene are exactly factual.
2. From a triangulation you cannot arrive at a circumference in a sphere, but a circumference in 2D. To test whether a 3D application would fit, he would need to find several other places with the same setup. For instance, by going to the coast of Spanish Sahara on the same latitude and work out the same calculation. And by the same token, he could have gone to Sukkur and do the same thing.
3. He should have done the same thing to the south, thereby adding to the concept of a 0 point at the summer solstice, and appearing angles on the higher latitudes, and on the lower latitudes.
4. It also presupposes that the man had a way to establish Syene and Alexandria to be on the same meridian as well.
5. That the distance indeed caused exactly 7 degrees and 12 minute of arc of shadow op de 21st. of june (being 1/50 of a circle).
6. Working out then the angles of certain stars visible on the same latitude, being the tropic of Cancer, on the same date, would eventually give evidence of two striking hypothesized features:

1.The known world can be suspected to be at least partly spherical.

He would then have to explain the strong current into the MED.

2. He could have postulated a concave earth with a concavity yet to be established since it was not yet known.

This is the problem with determinism. It presupposes all factors to the inquiry to be known and quantifiable. At least David Bohm leaves room for what is yet to be discovered.

It is amazing with the supposed knowledge Eratosthenes must have had, he forgot to do no 5.

But he did not and did not proof the earth is a sphere at all. Only partially at the meridian he chose.

That said, again referring back to the mariners. the math still holds. It would be fun to consider how to calculate and navigate on a concave earth.