To recap some prior points of mine:It took the USSR from 1945 to 1949 to produce an atomic device (not a deployable weapon) with a good deal of information from their spy system and the fact that they were no longer pouring resources in to supporting the battle against the Nazis. If the UK is not in the war, the Nazi threat is not perceived quite the same as OTL. It was the fear of a German bomb that had the USA throwing tons of money and resources art the problem, the use against Japan was just an extra benefit. Especially without LL the USSR is going to be hugely resource strained. If the Germans do better militarily with BARBAROSSA, which they will inevitably do if they and their allies are not devoting resources to fighting the Battle of the Atlantic, fighting in Yugoslavia and Greece, fighting in North Africa, not worrying about aircraft over Germany, this means the internal resources available to the USSR are going to be less than OTL.
The USSR has only so many resources, which include engineers and technicians and skilled workers. With the Nazis attacking the USSR needs them doing what they can to crank out more tanks, trucks, aircraft etc. Especially since all those trucks, radios, locomotives, gallons of AVGAS, SPAM, and more is not going to be flowing in to the USSR via LL. It took the USA, with essentially unlimited resources, and no attacks on the USA directly, over three years to produce a deliverable weapon and a delivery system (the B-29 and also the backup B-32). Expecting the USSR to do this under the circumstances of an enhanced BARBAROSSA and no allies, is simply unrealistic.
-I don't see the Soviets even contemplating building nuclear weapons in this time line unless they have:
-proof that they can actually be built. (Ie a successful test by the U.S.)
In this time line I would expect they would receive some assistance vis a vis the designs via espionage.
To add some other points:
I seem to recall some historical Soviet strategic bombing against German targets in ww2. In the absence of a western allied air campaign the Germans may not have heavy air defences. It doesn't seem impossible to me for the Soviets to deliver a special weapon assuming they can build one. (Which I agree is by no means a certainty.)
In terms of the circumstances in which the Soviets might pursue nuclear weapons,
I am envisioning a long more or less stalemated campaign between the Germans while they digest their new eastern holdings and the Soviets while they lick their wounds and build up their forces while continuing to keep pressure on the Germans.
I can see some efforts being made to clone a U.S. Nuclear weapon during this time period. If the Soviets are able to buy critical items from the west with their gold and likely other natural resources that may make things easier. If the Germans behave in their conquered eastern territories as they apparently planned on doing I suspect there will be a lot of sympathy in the west for the Soviet cause. This may translate into more help via espionage or perhaps other means.
On the other hand depending on the starting parameters the Soviets may win without needing to try and build nuclear weapons.