If America is not well prepared to do KGF, then I suggest moving the German subs to 91 the turn after the Italians take Gibraltar.
A big stack of subs can attack the American fleet, because America declares war at the end of its turn but cannot attack…if it isn’t ready, its declaring war in the face of a huge ambush…
or even invade America with a tran or two…the german planes can come through by landing Iceland or west indies.
Overall, trying to control the atlantic with german subs is not very functional. You can crunch UK income, use the subs as the cannon fodder for your airforce whenever it comes to threatening the UK or US buildup.
However, you will be spending most of your income on a unit that cannot defend, cannot take or block territory. Subs cannot really win you the game, so you are buying them in lieu of a surface navy or an armored attack. You may control the Atlantic for a while, but you certainly aren’t going to suffocate or stop America (perhaps only ward them off). A surface navy is far more functional in terms of flexibility–you can constantly threaten invasion, and even after UK and US get big, you can still sacrifice for one more chance to stop them.
Subs can also work this way, but they will all be sacrificed because they are a low attack low defense unit. 10-15 subs is not very intimidating, its very easy to strike from the air, and they cannot defend either by spreading out, holding 112… they also cannot hide in 113 as it is very easy to “slam the door” and trap the German fleet in the Baltic if it is trying to hold back…