Handcannoneer
It seems that only Handcannoneer is the main reason to have a strong mangonel, decreasing its ranged defense will obviously make it very vulnerable to Handcannoneer.
First I wonder what was the reason and purpose of this unit to have 35 damage, is it because the siege defense is 30?
If Handcannoneer is a big problem then nerf it, reduce its attack range to 3 and any other nerf you can think of and that way we solve all this