If nobody has made a special complicated regime for single-player as USA, then obviously no improvement in the protection of the Atlantic coast is worth waiting for.
But when making single-player regime for more complicated game as Britain, I ran into the same problem.
There was a need to strengthen the AI protection of North Africa, Italy and the Atlantic coast.
I used two ways to make it harder for the player to invade these areas:
1. Place the hard locked (so that the AI does not take them away) infantry divisions on the beaches and in places accessible to paratroopers.
2. Additional AI events that immediately put additional German tank armies for free in regions that have undergone landings.
And if the player lands earlier than the historical time, then Germany gets even more additional tank armies in the landing area.
Thus, the main problems for the player arise not in the process of landing, but after landing, when super-overboosted German tank armies rush to dump him into the sea.
If I would doing a complicated SP regime as the USA, I would use the same methods.