Isn't it all just solved with a simple model comparison?
Title. We have model A) flat earth and model B) round earth. We go a step away from reality and just compare a round object and a flat object rotating (we see the rotation) with some randomly scattered (but fixed) lights in the background. What's the biggest difference between those two models? For the observer on the flat earth there is one point where all the fixed lights rotate around. For the observer on the round earth there are two points were all the fixed lights rotate around. The moment you have two different points were all the fixed lights rotate around, you cannot be flat anymore. Those two conditions are mutually exclusive. This is our prediction from our simple model and now we look for clues in reality. And Tada. No flat earth.
How do we convince people that the earth is in fact round? Angles from the surface to those two points.
When the flat earthers say: Earth does not rotate, the stars do. Ask them where the difference is. Movement is relative.
How can anyone possible argue against that?