the best case for Nazi regime would be to stop after they had destroyed much of Soviet military and captured Leningrad? they could demand all the aircraft and naval vessels, as well as obviously grain and oil for "occupation costs" beyond anything they could have gained from marching on Ukraine and Caucasus? (cannot imagine a USSR that withdraws from the war would receive much if any Lend Lease aid? so their situation would be pretty dire and unable to reconstruct a modern military. they would closely resemble the KMT China regime with large occupied part of their country)
Then there would be residual Red Army formations in the Caucasus. their raiding and efforts at partisan warfare would be another incentive. If the Allies build up a army group in Persia and start probing north ...
Heres a minor question. Under the conditions of the OP, what residual Red Army could still exist east of the Volga? Even if there is a armistice how large a future threat would be a 'Russian' or communist state in that region be? Population, industry, withdrawn armies and weapons after the end of the Soviet debacle?
what if my scenario occurred and Soviets forced to terms? could their oilfields become target similar to French fleet after their armistice? (an Allied target)