Everything you are saying is simple ignorance of the model. They say that you can see at 35,000 feet in perfect weather with sufficient FOV, but I've never heard of anyone seeing it at that altitude.
Lots of people on the Concorde said they saw the curvature at 60,000 feet. At 100,000 feet, it starts to be barely discernible in cameras, and even at 300,000 feet, it's only starting to be obvious without a straight edge.
So, first of all, when you say there is no curvature at 100,000 feet, you are not using a straight edge, and you are not aware of how MUCH curvature their SHOULD be. Without the correct expectations, how could you possibly know when to see the curvature?
Here is the curvature at 100,000 feet. Barely discernable. Remember, this is not very high. YOu seem to think it is, but someone can jog that distance in a couple of hours. The earth is HUGE.
Here is an image from a high alt balloon at 100,000 feet taken by a flat earther. It's exactly the expected curvature for 100,000 feet, which is barely discernible, but measurable with a straight edge.
But you see the curvature all the time! Just going up in altitude and seeing further IS the curvature.
And seeing horizon drop IS the curvature. They are the curvature of the earth dropping away from you so that you can peek over the curve.
You can put an app on your phone and see horizon drop.. if you were interested in learning the truth.
RE: Question : At what height do we see curvature