Hitler's brass was pretty much right in with him about invading Russia though. Yes, there were a couple of people in logistics who were like "um, our tanks break down like every 5 kilometers; how the hell are we supposed to push 1,000 km in one season? Are you planning on buying a bunch of Shermans from the Americans?", but they were generally ignored. German intelligence on Russia was *horrible*, which I'm sure was in large part due to the purges but also had to be in large part because of gross incompetence by the Nazis as a whole. Like, there are contemporary intelligence reports about Russian officers that are basically "we're pretty sure this guy is in charge of this army and he probably has a staff that accompanies him". No "these are the people who are underneath him at the moment", not even "he has 14 attaches", basically "yes, this person is a general. We are pretty sure of that". The Germans also estimated that the Soviet Army was something like 1/3rd the size that it actually was and their estimate of how many tanks and artillery the USSR possessed was orders of magnitude lower than reality sometimes. This gets even more unconscionable when you realize that the Germans had actually been using Russian equipment and in some cases soldiers to train!
The USSR actually got really, really good by like 1942-3 or so at misleading the Germans in terms of where they were going to attack. Lest we forget, too, the Russians didn't merely throw their hordes at the Germans or whatever, they straight up encircled huge portions of their armies twice and destroyed them in detail. Yes, the purges hurt a lot, and yes, there was a good 2 weeks to a month in 1941 where the Germans were advancing as quickly as their army could move, but at the end of the day the Nazis got more or less wiped out by Russia, and not just because of the manpower differential, also because they were flat-out outfought on that front.
This is a lecture by David Glantz, a US military historian and actually *the* guy for the US who got access to Soviet accounts and basically debunked all the BS that the Germans told us after the war was over> I realize that outside of the US this may not be all that meaningful but it's worth noting that the US military absolutely does *not* fall in behind the wehraboos when it comes to the "Eastern Front" (which, by the way, try parsing that out with people who scream that only victors write history):