I don't think the WAllies getting air superiority is a given. First, is far cheaper to build a fighter than a four engine bomber. Second, while it takes time, Germany can switch from producing armaments for fighting in land to armaments to contesting the air (and stuff like 88mm antitank/flak guns don't even need to be retooled, they just get redeployed). Third, peace with the USSR likely means they get access to a lot of metals they lacked OTL, so they get more reliable jet engines. Four, while it also takes time, they can move factories further east, meaning the Allied bombers need to expend more time over Nazi airspace.
All in all, I think the WAllies will keep the fight in the periphery: North Africa, the Italian islands, Norway, maybe the Aegean and Greece while bidding their time for Project Manhattan to bear fruits. And then they begin to drop nuclear bombs on Germany at night. After 6-7 nuclear bombardments, some sort of peace is achieved. The main issue will be that the Allies will (understandably) want to occupy Germany and, without Wallies armies in mainland Europe, the Nazis won't easily budge, despite weekly nuclear attacks.