OP chose a hill 250 meters above sea level, and also has an antenna about 5m high. According to a calculator I found[1] that puts the distance of the horizon at roughly 57km. So plenty of range to still be in LOS of the transmitter.

[1] https://www.omnicalculator.com/physics/earth-curvature

(after posting this I thought of "liquid/atmosphere only" planets and how they are round so obviously my initial thought process on this was wrong, I thought water followed the bottom of its curved-earth container while trying to remain level on top)

This is nonsense. Do you believe that water forms a cliff at the edge of the beach so that its curvature tangent to the center of the body of water is zero? Hilarious

I'm fairly sure water follows the curvature of the earth...

meniscus please

Teach us more science facts, professor!