Churchill was dragging his feet in the mud. He didn’t like the soviets and he made that very clear. The allied powers also had the cairo and tehran conferences in late 1943 (well after Stalingrad) where Stalin was begging for the western front to open. Then it took another 7 months to actually do D-Day.
Look i dont know the month to month military capabilities of the western front. But what i do know is that the west was incentivized to delay invasion for as long as possible. Yes, part of that was military build-up, but part of that was also just letting the USSR and Nazi Germany tire each other out. And after the war, instead of recognizing the wins, losses and efforts the soviets made during the war, the US almost immediately jumped into cold war posturing.