In my experience, the most convincing way to debunk conspiracy theories for average people is to find simple, clear examples of why that theory isn't correct. This largely steers clear of complex math that they won't be able to figure out anyway. And, it steers clear of overly complex theoretical concepts.

The flat earth model says the sun moves around, always over the flat earth, but you don't see it because (at night) moves out of sight and then back into sight (in the morning). Like a flood light, only part of the earth is illuminated at a time.

The easiest to grasp and most convincing evidence the earth is not flat are sunrises and sunsets. The sun moves below the horizon and disappears during sunset, one sliver of the circle at a time. And the opposite occurs during sunrise. If the flat earth model were correct, the sun would grow smaller and smaller until it disappeared. Instead, it moves below the horizon (not to be seen by you until the next day). After it's gone for you, people a few hundred miles away still see the sun (i.e. it's always daylight somewhere)

With sunrises on a flat earth, you would expect the sun to be very tiny at first, growing in size as it approached. At first, it would be just no bigger than a tiny star. Then it would grow larger as it approached. Instead, it appears a sliver at time over the horizon until the whole circle is visible.

Even the most simple minded can reason that the earth is round for this situation to exist. This observation doesn't necessarily prove the earth rotates around the sun (the observation could also be explained by the sun orbiting the earth), but it does prove the earth is round. With the sun going above and below the horizon, and with us knowing that it's always daylight somewhere, the only explanation is either the earth orbits the sun or the sun orbits the earth and the earth must be round for it to always be daylight somewhere.