Alot depends on if this is Stalin jumping off the deep end on his own though: the French certainly aren't going to be staging any major offensives that might draw German military resources to the Western front or forcing them out of the protective range of the Magenot Line at the cost of French blood and treasure merely for the sake of a power-grab on Moscow's part;
Sure. But that matters little. Germany's still going to have to devote significant assets to the west to keep that cost of French blood and treasure so high and that portion of assets will only rise,as Anglo-French strength gains and German strength wanes.
Plus, the Germans have more flexability in staging a strategic offensive
After the initial lunge? No, they really don't. Without the loot from western Europe and without the imports of Soviet fuel, rubber, and raw materials they lack the logistics for the sort of deep offensive that would be needed just to gut Soviet military-industrial complex as they did IOTL. Logistics places rather severe limits on how deep they can go and the Soviets, for all their faults, will learn fast. Their mobile forces will also have been badly depleted by the necessity of continuing to campaign without rest past the Polish campaign (and Soviet resistance during the initial classes, even though inept, will still be quite heavy), meaning even attempting to repeat their 1940 performance is rather out of the question. They'll probably still be able to breach segments of the Stalin line in 1940 if they commit too it, but lack the strength for real exploitation necessary to destroy the Soviet forces manning it meaning the Soviets just fall back to a new defense line a hundred kilometers further back.
and now its the Soviets, rather than they, who have to worry about domestically justifying the war and limiting internal dissent.
Not that hard given that this is Soviet Russia. The country had been living in psychological preparedness for a war with the Capitalist West since the revolution and any lingering doubts will rapidly be expelled as German behavior in western Belarus and Ukraine becomes known.
In that case, I woulden't count out the German war effort lasting as long if not longer than IRL,
I would. Without Soviet imports, Germany will be facing a literal famine by the winter of 1940-41... much less the consequences of what the lack of imports of Soviet raw material and fuel does to their industry.
particularly in the (I consider likely) event of a Japanese DOW on the USSR in response and the non-agression stance taking place between the Western Powers and Japanese instead in response.
The Japanese forces in the East, which had just received a very pointed drubbing at Khalkin Ghol, are even worse positioned for a deep offensive into the Far East then those of the Germans. They are also totally incapable of reaching any areas that actually matter to the Soviet war economy. At best, the Soviets commit the minimal forces to hold them while finishing off the Germans and then transfer major forces eastward in '42 or '43 to crush them.