A ship disappearing from the bottom first is an effect of Earth’s curvature. It is not a direct observation of the curvature itself. We can only directly see the curvature from a significant altitude, not from near the surface.
Flat-Earthers present this reality as if it is a dilemma: 1. Earth is too big for us to see the curvature, but 2. We can see ships go over the curvature. It is a false dilemma. Ships disappearing from the bottom first is an effect of Earth’s curvature. Witnessing such an effect of Earth’s curvature is different from witnessing the curvature itself.
Observing the curvature of the Earth means directly seeing the curved horizon. Directly observing Earth’s curvature is easier if the observer is at a higher position. On the other hand, ships going over the horizon is the result of an observation. Earth’s curvature is the fact inferred from the observation.
Flat-Earthers like to dismiss the appearance of ships going over the curvature by pointing out that the horizon appears flat. They are wrong. The two observations require different conditions.
Directly observing the curvature is easier if the observer’s position is higher, and taken using a wide-angle camera lens. Observing ships going over the horizon is easier if the observer is closer to the surface, and using a long telescope or a telephoto camera lens. It is practically impossible to observe Earth’s curvature and a ship going over the curvature at the same time.