I would think if you’d want to fix the role of submarines, then fixes should be directed to possibly to biggest impact subs had on the war - attacking shipping and supply lines.
How about a “SBR-type” rule for subs in key sea zones? Maybe against shipments to UK and Japan only since they were most dependent on outside resources? Destroyers protecting these sea zones get a “AA” type role to defend so that subs have a chance to live and there is incentive to build subs? I would think that this could open up the Atlantic and Pacific a little more for something more than just sea unit stacking.
Just a thought.