Part of the problem is positional any Axis subs being built are far from the enemy’s production and they have to try to move into the flow of Allied fleets. There are situations that the enemy loses his destroyer protection but since you cant target destroyers specifically, its more of a tactical thing than a strategy of emphasizing sub/economic warfare.
So there are really two problems–the subs are like strat bombers, they can position fight or destroy money but not both at the same time. They’re not defensive units either, to keep them cheap and balanced, but this means they have no protection.
I don’t think modding combat or subs is really the best option, modding the map and how economic warfare works would be easier
(so make SZ 100, SZ 112, 79 38 5 4 2 55 61 56 into Convoy zones)
(90, 103, 107, 117, 124 12 become USA/UK income convoy zones where Axis subs can destroy general income of either side)
(Allow the ships to attack NO income relevant to that terr so a Japanese sub in 45 attacks up to 5 of ANZAC income etc etc)