As an alternative, change their functionality.
For example, make it so that the cartographer only reveals the area in which they are located, instead of the entire map.
Or, make it so that the cartographer hides the entire map from the opponent, or a portion near the cartographer, for 5000, for example.
And make the hill fort require double the upgrade cost, like in Heroes II. Maybe, in addition to level 7, let them upgrade the rest themselves.
Triple the cost, and let them upgrade whatever they want!!!

