I think ARTY is not very effective in breaking through prepared defences currently in game, as it relies on suppression effect and direct hit to help soften those defences instead of destroying defence structures.
So I propose to create an entrenchment module: an entrenchment value is created for terrain, and a stationary unit can increase this value at its position. Rest mode can affect the build up. ARTY and airstrikes reduce this value, down to preset base level(s). And a formula can transform this value to hit probability. A simpler solution can be, just let units have that value, and the formula converts the value into hit probability reduction, which is added to the terrain hit probability.
So I propose to create an entrenchment module: an entrenchment value is created for terrain, and a stationary unit can increase this value at its position. Rest mode can affect the build up. ARTY and airstrikes reduce this value, down to preset base level(s). And a formula can transform this value to hit probability. A simpler solution can be, just let units have that value, and the formula converts the value into hit probability reduction, which is added to the terrain hit probability.