Couldn't have been much worse than OTL;
A grinding war of attrition on top of all that other stuff? Despite all of the pounding the German homeland was taking most of the German army was still in the field, deployed against the WAllied lodgements in France, Greece, and Italy even if they were weakening fast from the lack of replacements.
And hey, maybe taking the burden of breaking most of the German army would have put the Soviets in a more favorable light in the western eyes, averting the Cold War. Then again, given the sheer gap in views between the Russians and the west, I'm kinda dubious on that.
if the Soviets were so messed up by the purges
Except it's pretty clear they were not
that messed up by the purges, otherwise they would have never rebuilt the Red Army.
maybe the Germans could have toppled Stalin because of how unpopular he was and gotten all the resources that sank them IOTL.
Given how poorly thought out Barbarossa apparently was? Fat chance. And Stalin had pretty well cowed any sort of dissent within the Soviet Union. Even the recently annexed frontier region was thoroughly cowed by '44-'45.
Occupying the USSR would be a worse meatgrinder that Greece ever was: there would be no way to extract resources from there. It's a wonder that the Nazis got as much from the MR Pact.
It's pretty clear that the MR-Pact wasn't a one-sided deal though. From '42 on, Stalin kept making Hitler pay exorbitantly more for the flow of raw materials.