swift horizon
Make the current artillery in the game into a commander support ability. Say, 450 ammo for a 21-shell barrage (7 a piece), but increase the spread to something approaching the Katyusha’s AoE, and doesn’t require players.
As for actual mortar squads, have a team leader and mortarmen equipped with carbines/SMG. They have set ammo on their person to operate a mortar that can be refilled by an ammunition dump constructible (that would also resupply tanks and infantry) or a couple round per explosive crate, and have a max range of say 600m. The team leader can deploy the mortar, but doesn’t carry ammo.