@meterpaffay What you are doing is totally viable and many players trade with UK but not US.
The US masses units in Finland then they try to take Karelia. From there US moves its large stack into Baltic states setting up for the capture of Berlin.
The Baltic stack might not be all US forces but combined forces of US, UK and Russia. So Allies may be able to hold Baltic but not have enough to attack Berlin from there alone.
This is ok. At this point you can have UK and US land in France together which puts Germany in a double bind. While it may have enough to recapture France, it may not have enough left in Berlin to defend vs forces UK and US can bring to Berlin and from Baltic if it does attack France.
While I think there are some strategies even stronger than this, this method is good and it works.
As to your question Germany should not commit too much of its forces to defend France in the early parts of the game because it wants to advance as many troops to Russia as it can instead.
There are times when Germany can defend France with few units, like 4 in the early game and you are right, it’s good for Germany to deny this income from Allies so they can’t get a IPC boost used to further build up expensive navy they need.
But other than that it’s kind of wasteful for Germany to commit 10 or more units it may need to defend France from Allies. Think of it this way, Germany can build 10 units in Berlin. So 10 or more units is basically a full turn worth of production being used to defend France instead of moving those units to the front lines.
Infantry are the most cost effective unit in the game. They should be the bulk of almost any Army. They are slow however. It takes 3 rounds for them to move adjacent to Moscow from Berlin. So if they are in France instead, it’s a lot of time wasted.
If the Axis is about to win by having 9 Victory Cities, then it makes sense to stack and hold France if it is one of those 9. But not really before then.
Attacking with massive number superiority is much more effective than fair attacks. So having 20 or more units in Berlin is actually better than having 10 in France 10 in Berlin. As now Germany can attack with better odds and take fewer casualties when it does take back France. If it can capture France with enough units surviving that Allies cannot take it back, then Allies land somewhere else and Germany attacks there with its large army. Then keep doing that instead of losing more units when trading. Some people call this pivot stacking. Because the same large core of units keeps attacking adjacent territories pivoting its position, but being strong enough to not be counter attacked.