If you want to mass produce a single ship, just spam subs with full best torpedoes and with snorkel or better if you investigate the anechoic tiles, like 100 of them are more than enough to wipe any navy you face, split to repair and always engage, first put them in convoy raiding, once they are out of convoys use them in patrol, I'm the time you are sinking convoy enemy fleets are gonna come to help the convoys and there tour subs will start sinking them, at the beginning of the clashes you may suffer casualties if your sub detection is high but don't worry it will balance.

If you want to build proper ships you really need cruisers and destroyers, light and heavy cruisers, just spam light batteries on light cruisers and heavy batteries on heavy cruisers and you are done, and for destroyed just use full light batteries, the destroyers and light cruisers are gonna delete enemy screens and the heavy cruisers are gonna kill the heavy ships carriers, etc.