It has already done. Countless flat earth believer have sent high altitude ballons and proved the earth is flat.
More question?
You completely ignored his first question about the sun. Also how exactly does flying a weather balloon and seeing the curvature of the earth prove that it is flat?
Yes, I ignored the first question, because I like to ignore something.
In other say; I don't use wiki of flat earth as an argument, because it is a nonsence. High altitude ballons are shows a flat earth. Curvature or not, only related with fish eye or go pro camera is used or not. That's all.
In other say, if you look flat, see flat the horizon. If you see round, you see round the horizon.
In short, the shape of the earth is relevant with what you want to see it.