I fail to believe that flat earthers really hold the opinion that the earth is flat. I think they are 1. trolling or 2. just want something to argue about to fill the void that is their lives.
Unfortunately there are a lot of people who actually do believe it is flat, let's face it people are not rational, they have all sorts of cognitive fallacies
Just go on YouTube You'll see a bunch of dudes who you can tell are a little bit off and they're going on and on about it
Anyway I think the easiest way to settle it is just to say why can't you see other skyscrapers from the highest skyscrapers in USA, like why can't you see from New York to Chicago to LA when you are way elevated
Same thing about why can you see the sun and the Moon on the horizon, yet you can't see the tallest mountain ranges when you are more than a few hundred miles away from them
Where did they suddenly vanish to? We're talkin about four and five mile of elevation with certain mountain ranges... if the Earth was flat you would be able to see them from thousands of miles away