It seems to me it would be best to revert this change as soon as possible (before the expansion). I did not hear a lot of complaints about support brigades before they changed them. To introduce true realism to damage dealt to support brigades would be quite complicated. Letting artillery regiments get smashed on the offensive so that the entire division ceases attacking is not the way to go about it. Nor is it a good idea to let people's and AI's port defence units in very expensive dual type fortifications surrender because their engineers get spooked.
No unit should retreat, surrender or break off the attack until all combat type regiments have broken down. Maybe this was not how it was before, but because the support brigades were relatively protected (as they in many cases should be!) it was never a problem. It worked reasonably well before. It should not be too complicated to go back.
For realism, different type of support regiments should be subject to damage, and indeed be employed at all, depending on the combat situation. All regiments should be subject to damage from air attacks. The damage should be distributed according to the relative size of the regiment. Relative vulnerability to air attack is already simulated in the stats. If you are one of the people arguing targeting support brigades specifically makes sense, you should suggest they alter some of the currently useless air doctrine target selection technologies to reflect this, or add another type of air mission.
Artillery should not take damage from ground combat while attacking. Counter-battery fire was not all that effective and rather than modelling a very limited special damage for this, it would be better to ignore it.
AA and AT regiments should not be employed at all in offensive operations. True, when attacking fortified positions, they could be instrumental in cracking the shell of the enemy. You will be hard pressed to find examples of soldiers charging an enemy tank formation with AT guns though. It is just too cumbersome. All in all the oddity of employing these very defensive units in an attack outweighs the need to simulate they sometimes were used that way.
Artillery and AA regiments should suffer fewer casualties than the true front-line regiments in a defensive combat as well. Commanders and soldiers knew those units were vulnerable and strived to protect them, both through positioning and actual tactical combat decisions. It is true some of these units were sometimes employed right at the front for direct fire against enemy ground forces, but those were desperate decisions or a result of severe failures. Keep in mind artillery and AA regiments are inferior in this role to for example AT regiments. Commaders of this era were aware of this too.