To me the US is very much support. It just takes WAY too long to get the transport chain in place (turn 4 or 5 before it is stably delivering 3 or 4 tran of stuff). The UK to me is doing most of the offensive heavywork against Germany. It is very quickly within range of dropping 4 tran of stuff into Europe so it tends to have the most troops in Europe, and anything that you built can be used right next turn instead of waiting a few. The US is good at liberating Africa and throwing in little bits of support troops for the first few turns, and even later is mostly just waiting for the UK to attack first while holding territories for them.
The trick with Germany in a KGF is to always, no matter what, buy at least 1 offensive unit each turn. ART if cash is tight, or an ARM since it also boosts defense.
I have a bit of a different view here. To me, Germany should just buy infantry nonstop after any initial naval purchase. Germany is playing a defensive game hands down. Remember that it starts with 6 fighters, 1 bomber, and 9 tanks on the mainland. It could be very well 5 fighters, 1 bomber, and 7 tanks on the mainland after the Russian attack, but that is plenty of offensive units to trade territories or crush an early landing in W. Europe easily. You just can’t have enough defense as Germany, because you have to hold out until Russia bites it. I do see where the 1 or 2 leftoever IPCs can be converted into art/tanks, and it is useful, but I’d rather just save every last bit and funnel it into the most efficient defense available, while using the existing network to do the heavy work.