Been having a think about this. I reckon the best solutions might be to let the armour value/health/unit type/breakthrough value of a unit/formation negate part or all of the fortification value of an entrenched unit/formation, have ineffective weapons not make the defensive unit's tonnage count for the purposes of breakthroughs, and/or have breakthrough attacks not count fortification bonuses.
Explanation: Right now, we have a WW1 scenario of fully entrenched everything. Ignoring the fact that the attackers can choose their landing spot when they choose to invade, how do you beat a massively entrenched position? Go around, be armoured enough that their weapons are ineffective, cause a breakthrough and negate the defensive advantage of the whole position, and bring guns that can break the fortifications.
Drilling down to the core of the problem: I can make a force of nothing but CAP pillboxes, entrench them in a steppe, and stall a heavy tank division for months because the heavy tanks can't hit the pillboxes. It makes little sense that a HAV weapon can't render fortifications effectively meaningless, but it makes even less sense that the pillboxes are 99% effective in stopping a breakthrough. Why can't the tanks just steamroll it, or go around? It's not like the CAP can threaten the tanks very much.
Better worded answer: Let the breakthrough value of a unit counteract hostile fortification bonuses, allow support units to increase the breakthrough value of the supported unit to simulate airstrikes/orbital strikes opening holes in a line, perhaps throw in a training for infantry to increase the breakthrough value for infantry units, then make breakthroughs totally ignore fortification bonuses.
Results: Heavy armour now slays steppe pillboxes. The bigger the tank, the better it is at flattening them. Entrenching infantry in a field WW1 style now has limited effect against tanks, as it should. Vietnam can still happen, thanks to the hitrate modifiers for infantry focused terrains making vehicles suck in those environments.