I think if the Americans had hit anything more than one or two cities in Germany itself the US would have had a even harsher peace deal and less independence than it was given -so maybe one city in Germany itself and one or two in German annexed territories ? OTOH I really think its likely that under Himmler the Germans would have nuked an American city (or British one during their revolt) to punish the effort- ie Atlanta as a ding at Maddox as welll ?I generally agree with your assessment. The US remaining behind the Germans seems plausible, but not way far behind (not like how MITHC had Japan so far behind the Reich...that was almost too much). If I were writing a TL that was freestanding and not trying to fill in the gaps of a preexisting book, I'd be more inclined to have a WWIII between the Reich and the USA that does have the Reich getting more heat even if it does manage to pull ahead in the end.
That said, your post has made me wonder if it would be able to fit one or two such City Busters hitting the Reich. Turtledove has Henrich and his friend Willi discussing the US, and they make mention of how the USA might have been a formidable foe, and that their fathers likely worried about them in earnest. Obviously, in the book, Berlin doesn't get it. But maybe some smaller cities? Bonn? Dresden? I don't think either was mentioned in the book. Maybe the Americans are able to get in a few rounds before they are forced to surrender.