About the navy: Dont mix subs with non subs

I usually have my subs on packs of 4-8 units (and name them sea wolf packs) because I can cover more regions like that and/ or more packs per region (more dispersed hunting packs -> higher chance of engaging convoys)

For ship fleets in my experience doomstacks of many ships are better for destroying the enemy navy, but some countries like UK have a spread large overseas territory to defend and this is not an option (unless you give up your resource rich colonies)

Dont put more than 4 carriers in the same fleet, in battle they can suffer an "overcrowded air space" penalty

Carriers are the best ships imo, but you have to produce planes for their air wings and need screen ships to escort them

Use your mainland air wings to support your navy in regions like the mediterranean or north seas, when your fleet engages the enemy navy send tons of NAV bombers or CAS to the region where the battle is raging and with good weather they will find and bomb the enemy ships