I'd like to argue that G43s are not a solid buff to the infantry because grenadiers already have the option to go for lmg42s and pgren stgs are great. G43s just provide you with a possibility for a different playstyle.
I understand where you are coming with this, but I think that G43s in their current state are nowhere near as good of an infantry upgrade as the ability to lay sandbags and mines with mainline infantry from CP0. Lightning war doctrine is balanced if you compare it to the Heavy Cavalry Company.
Well the state of the G43 is simply a result of the power creep and the continues buffs other units/abilities got.
Imo the ability should add the an upgrade to HQ costing some fuel that would allow Grenadier and possibly PG to upgrade with 5 entity and upgrade with 4+1 G43 than now are closer to M1 for Gren and m1 carbine for PG. That would solidify it as different play style.
As for "relief infatry" I would change the mechanics, Longer duration but instead of the classical Ostruppen you get an new squad of "Ostruppen" with target size 1 and no cover mechanics that can marge but not reinforce. Their job to replace front-line losses on the squads that are fighting. One could even add as small heal (4 hp?) when merging.
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Vipper's Mechanized Repair Team sounds interesting but, I'm curious about the mechanics of it. Are they tied to the half-track or can they wander the map on their own?
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The unit I suggested as passenger is a six men repair team with very poor damage and it exist in one of the FOW mission.
But I guess one could redesign the 250 to become a mobile repair station, Pioneer could passenger that automatically repair once the 250 goes to hull down.