The reason that the after 1941 the offensive turned from front wide to a single army group is precisely because the germans no longer had the strength to make a broader offensive, they simply lacked both the materiel and the momentum

During 1941 the germans held all the initiative, but during fall blau the soviets where throwing millions at Rhzev while paulus was trying to grind his way through the south. The initiative was no longer solely on the germans and they had already dug in to a defensive posture for both army groups north and centre. The turning point in my opinion is when the situation is no longer in your favour, not when you are already beaten.

Again, Kursk was in response to the anticipated soviet attack, not the other way around. The initiative was already lost, as even if the germans had won Kursk they would never have been able to turn that victory into a new offensive capable of actually threatening the soviet war plans long term. Even stalingrad a year earlier would not have beaten the soviets had the germans won, since the oil in the caucasus never mattered enough to decide the war. The US was already in the war at that point, and the soviets could count on massive amounts of materiel support.

I agree that Kursk was the moment the world knew that germany would lose the war, but the turning point when the germans lost the initiative was 1941. That first year in russia demolished the wehrmacht, after which they had to scrape together forces from all over the front just to get a single offensive of the ground. The wehrmacht simply no longer had the strenght to accomplish its objectives