Yes, back on topic, i get your ideas.
For mortar pit, let said 250mp for the pit with 1 mortar operational, then another 100mp for the second mortar. I brought up this idea a while ago in the other forum.
But, for bofor and 17 pdr, i think it will be tricky. Brace should be removed if possible and may be replace with something else. May be an upgrade to unlock stand fast as toggle ability.
As long as things come with tech:
One could add other abilities for the mortar like different munition and longer range (they are already in the game actually.)
Stand fast imo could easily be reintroduced but with a cost in MU or UP so that there is some cost attached to it.
So can be brace.
In addition the HP and armor of this emplacements could also have upgrades so that player can have more control of how much he want to invest on them.