How do you know the Earth is round? Recently there was an expedition to the South pole, that cost 35k per head, for flat eathers to go observe the 24 hour sun. It's called The Final Experiment.
I've devised a similar experiment that should be much cheaper than 35k. It should, in my opinion, be convincing to any honest flat earther if carried out. The plan is this:
Visit 4 destinations, 2 in the north and 2 in the south. Each hemisphere of our planet has night time vision of what's called a Celestial Pole, enough is just the part of the night sky corresponding to our axis of rotation. In the north, the celestial pole is very near to the North Star, and in the south it's very near to the Southern Cross.
So, the 4 destinations could be Canada, England, South Brazil and South Africa. Obviously they don't have to be those exact places, but they're a good example.
At each place, you set up a camera on a tripod to observe that celestial pole. You'll use that footage to create time lapses like this:
https://youtu.be/TZOg8EPJ_yk?si=Zryt1GUcldohiFpu
The reason you want 2 places in each hemisphere is to handle all possible objections. A flat earther can easily explain the celestial pole of the north. But to explain not just one, but two celestial poles in the south is actually much harder. The firmament would have to be rotating from east to west in order for the southern celestial pole to go from South Africa to Brazil - but the time lapse would show that in fact the southern celestial pole is not moving east to west, but is quite stationary.
Now of course i know this wouldn't actually convince them, but the theory crafting about it is just fun for me. This experiment would reduce the cost from 35k to something closer to 5k.