Here is something that just completely blows Flat Earth out of the water, and this is completely observable to everyone on the face of the Earth. The Earth casts a shadow on the moon, we know this because we can see it on certain nights when the sky is clear and the moon is visible. We can also see that during certain phases, whether it be waning or waxing, the shadow appears to have a curve to it. This is huge, because one, if the Earth were completely flat as they claim, then there would be no sunset, hence, the Earth could not cast a shadow on the moon, because the moon would be continuously full and lit by the sun, since nothing would be in the way to block or cast a shadow, and two, in order for there to be a shadow cast, supposing the Earth is flat, the sun would have to go down the underside of the Earth, with the moon on top, visible to us. That doesn't happen, because we also observe that on some days when the sun is out, the moon is nowhere visible.
Also, if the Earth were flat, even if the sun somehow managed to cast a shadow of Earth on the moon, it would appear thin or straight. But on a spherical object, no matter at what angle you shine a light on it, behind the sphere, the shadow ALWAYS has a curve to it. That is something you can even test for yourself. Take a tennis ball and flash light, and shine a light on the ball at a white wall, at any angle, and you'll see a round shadow, the same thing we observe with Earth's shadow on the moon. It's not a matter of perception, it's a matter of the Earth having curve to it, and the sun sets and shines from the other side of the world.
Everytime I try to present this to someone who believes in Flat Earth, even some who "claim" to be Christian, I usually get an insult thrown my way or I get tuned out, but I have noticed, that once I say I believe in Flat Earth, they are completely welcoming and throw SO many different models and ideas about the Flat Earth out there. Biggest part of me thinks Flat Earth might be a deceiving cult, and an attention grabbing scheme.