No, it’s less complex than you think.
All you have to do is to move the cargo on the convoy route and protect it if there’s a sub near by. The rules of transport and cargo are almost the same.
In most AA game, UK and Japan are too stronger.
During WWII, Germany tried to cut the UK supply.
US make the same thing at the beginning of 1943 against Japan.
If UK or Japan player build a thousand of tank,plane and other materiels each turn even if they lost their fleet and the control of the ocean. That doesn’t make any sense!
For the warship cost.
Each country has their own cost.
ex: Uboat = 5, USA,UK sub = 8, Italy 9 etc…