I mean if you want u-boots to be more important, just build more u-boots bro. It's entirely doable to entirely shut America out of the war by saturating the ocean with enough submarines.
Is it really?
I am an u-boot autist. I have an entire bookshelf dedicated solely to the memoirs of u-boot captains. Großadmiral Karl Dönitz's "Zehn Jahre, Zwanzig Tage" is one of my favorite books of all time. I'm as autistic about u-boots as one can be, and in any playthrough of HOI4 I make a shitton of u-boots. And it's never enough. First of all, I don't think that it's possible to completely starve the UK out of resources, while it should be (IRL, at one point the UK almost had their oil reserves depleted, they had reserves for less than a month and if that would get used up, they would suffer industrial collapse; meanwhile in the game, at all time they get magical oil). And yes, you can shut America out of the war *in Europe*, it is easy to do, but you cannot totally ubootmog America into submission after you conquer Western Europe (which in my opinion is basically the tutorial section of a Reich campaign, you should not have any problems with securing everything from Germany to Portugal - I usually leave out Poland for the Soviets in the early game because I have no need of it).