“You can see variations in the surface, hills and valleys and so forth, but not earth curvature from that height.”
I’m guessing you live in hill country. I lived in hill or mountain country most of my life. It was only when I moved to the coast that I really noticed the curvature of the Earth (without being in an airplane). It actually took me by surprise. Even the scariest ocean waves seem smooth, compared to hills and valleys.
A simple straight-edge is all that’s required to confirm curvature. Hold it out to so that it appears to touch the horizon — the curvature is plain to see.
BTW, according to this handy calculator, at a 200’ elevation, the horizon is 17.3 miles away.
http://www.ringbell.co.uk/info/hdist.htm