I would suggest changing Support unit Defensive calculations from using just Fortification to dynamic system, where it uses Fortification if it supports Units in defence, and to Hit mod if it supports units during attack. It might better reflect combat for role play, as Artillery (Without colossal range, represented in game with Heavy Arty in rear echelons) would have to move with the attacking units, thus leaving the comfort of their entrenched positions. Also it would give more reason to Mechanize your Arty, as now the Hit mod would help them avoid getting hit while Supporting the front line.
Edit. This would be especially Helpful during planetary assaults, where theoretically there is not much time to dig in your arty.