Here’s my take. 86 all the specialized rules for subs, destroyers and transports. Treat them like any other unit. Allow subs to make an “SBR” in convoy zones. Allow Destroyers in those zones to act as “AAA” if applicable.
First bump the sub defense back up to 2.
In the case of non-global games use SZs adjacent to ICs as convoy zones. Keep the sub’s range the same but allow them to return to a friendly SZ after a “commerce raid”, remember they must have range just like an aircraft unit.
Allow planes to take out subs and allow subs to fire back assuming they would have AAA capability, which not exactly historical, but did happen and is somewhat plausible. Same for TRNs.
Let TRNs defend at 1 all other OOB rules apply.
To balance it out let DDs support amphibious assault for one round 1:1 infantry, CAs 1:2, and BBs 1:3. All other OOB rules would apply.
Third, let APs, DDs, and SS just be one price 8 IPCs. Their abilities and advantages amongst each other at the same price-point (IMO) negate much of the debate of “fodder/balance/etc.” BS.
Just my take. Have at it guys :-)