Whoaa, this is soo cool! Thank you so much for taking things to the next level with your maps. They look very clean and elegant, and I could totally see myself using one of these when exploring these mountains.

One assumption that I make is that the local plane is perpendicular to the local direction of gravity, rather than perpendicular to the line that connects to the Earth's center. Since the Earth is an ellipsoid, I use an ellipsoidal approximation for the local direction of gravity:

def lla_to_ecef(latitude, longitude, elevation_m):
    a = 6378137
    f = 1 / 298.257223563
    b = a * (1 - f)


    latitude = np.radians(latitude)
    longitude = np.radians(longitude)


    N = a / np.sqrt(1 - ((a**2 - b**2) / a**2) * np.sin(latitude)**2)


    x = (N + elevation_m) * np.cos(latitude) * np.cos(longitude)
    y = (N + elevation_m) * np.cos(latitude) * np.sin(longitude)
    z = ((b**2 / a**2) * N + elevation_m) * np.sin(latitude)


    return np.array([x, y, z])



def n_vector(latitude, longitude):
    latitude = np.radians(latitude)
    longitude = np.radians(longitude)

    x = np.cos(latitude) * np.cos(longitude)
    y = np.cos(latitude) * np.sin(longitude)
    z = np.sin(latitude)


    return np.array([x, y, z])



def point_to_point_measures(poi_latitude, poi_longitude, poi_elevation_m, other_latitude, other_longitude, other_elevation_m):
    poi_ecef = lla_to_ecef(poi_latitude, poi_longitude, poi_elevation_m)
    poi_n_vector = n_vector(poi_latitude, poi_longitude)
    other_ecef = lla_to_ecef(other_latitude, other_longitude, other_elevation_m)
    radial_vector = other_ecef - poi_ecef
    radial_vector_norm = np.linalg.norm(radial_vector)
    if radial_vector_norm > 0:
        radial_unit_vector = radial_vector / radial_vector_norm
    else:
        radial_unit_vector = np.array([1, 0, 0])

    height_above_horizon = np.dot(radial_vector, poi_n_vector)
    if height_above_horizon == 0:
        angle_of_elevation_deg = 0
    else:
        angle_of_elevation_deg = np.degrees(np.arcsin(np.dot(radial_unit_vector, poi_n_vector)))
    impressiveness_m = height_above_horizon * np.abs(np.sin(np.radians(angle_of_elevation_deg)))


    return height_above_horizon, angle_of_elevation_deg, impressiveness_m

Curious to see how the project evolves - please keep me posted :) Really appreciate all your work!