The main thing to take from the Transit of Venus/Mercury problem is that if these objects orbit the sun, they have to do to at an angle based on the paths they draw across the sky and across the sun when they are directly observable.
If this is the case, then with the hovering sun in the Flat Earth model, they have to pass between the Sun and the Earth In the gap of between 64 and 3000 kilometers or miles, depending on who you ask. (No reliable or consistent figure or unit for the height of the sun has ever been given and the most extreme assertions range from Sandokhan's 32km to Bishop's 9.3 million km)
If these objects were performing these close-pass orbits in the way that is observed, they would be visible from multiple latitudes at all times and cause appreciable perturbations of the atmosphere, as well as casting shadows in the spotlight Sun's light cone.
No flat cosmological model accounts for this or the motion and visibility of any other planet in the sky - and don't even get me started on the stars.
The Transit Problem is one of the more crucial points for Flat Earth theorists to explain if they want to use the "just look outside" defense.
We're looking. We are obviously not seeing what you are.