Measure down to the water at each end, then use a level laser to check for curvature.

They have already done that multiple times. The problem is that lasers "curve" in the air according to mainstream science. Your test would not reveal any curvature.

I never got the argument with the fly nose down either. Assuming the earth is round, why would you need to put your nose down? It makes no sense. Higher layers are less dense, so you would automatically curve with no need for a correction. As a pilot, have you ever seen the earth curve? With your own eyes?

Of the 6-7 flatters I've seriously engaged, not a single one would touch the arguments in the debunking videos in this playlist below.

The first video is basically him debunking by assuming the round earth model to be true. He is not doing any experiments to demonstrate a round earth. Do you get the difference? Not trying to be condescending, so I hope you don't take it as such but it is an important distinction and it is one where most of these "debunks" go wrong.

This video engages with your "two things flatters say". https://www.youtube.com/watch?v=WmqE2cRFbPU

There is a response to the 10 challenges to flatters on bitchute but I haven't watched it so I can't say if it is any good and it is several hours long. I don't know the channel either.

Is there a specific round earth argument or two that you think is very especially convincing?

If there is such a thing as a professional flat earther, I'd love to see them debate this guy, but that would essentially be a suicide.

Does Dave ever, in any of the videos, demonstrate that the earth curves?