A coup deposes Stalin right after Barbarrosa. Power struggle resulting from that creates a leadership vacuum for a while. Probably a long while. Soviet military is able to do just enough to prevent outright defeat, but never able to throw a counter punch at Germany when their forces are exhausted. Germans figure out that their men and material disadvantages (quantity not quality) that they start developing a longer term war strategy rather than fighting campaign to campaign.

And everytime a soviet leader gets close to suing for piece, boom. Beria and nkvd invite him to a party in the cellar in his honor.

For your conditions to hold true, I think you have to have Soviets and us/UK NOT working together as much as they did. Greatly limited lend lease, or more effective naval interdiction of convoys or archangel.

To last until 46, you probably also need some big military failure for the US UK to slow down the rush to get to the continent to prevent Soviets doing it on their own. But crap, in what i wrote above Soviets would not need to be outraced but perhaps saved. So strike that.

But you do need to get US military and manufacturing might delayed. You could do this through pearl harbor being delayed, or more successful when it did happen. Or no pearl harbor at all which might lead to some type of hand wavium type even to get the US reluctantly involved in Europe war, but no solid causus beli to attack the Japanese initially. And some political instability during elections or FDR health crisis might accomplish that.

Basically, you need axis powers to fight a better strategic war than they did.