I get it, you're using Finland as an example. The thing is central Poland is not very mountainous and the winters are not particularly harsh there and the Germans dont have the home advantage because they themselves are invaders. Where would the German tanks and aircraft come from to defend in the East? They're already in use in the West. Even if the Red Army is a total mess the Red Airforce would rule the skies and pound everything from Kiev to Dresden back to the stone age.
The soldiers dont have to execute brilliant maneuvers, they just need to shoot their guns, the Germans even being 10 times better than the Soviets man vs man still means a crushing Soviet victory.
So you mean full frontal assault look at Finland age Mg42s landmines and a modern army and watch.Again the Germans don't need to commit massive forces to this take some 2nd rate divisions guarding the Siegfried line as well as whatever units that are not needed and can be spared integrate a handful of veteran formations into them have pioneer units set up defenses in suitable locations move the old pre dreads the Germans have and dig dig dig. The Red army is ill prepared and ill supplied they would eventually breakthrough yes after horrendous casualties and after anywhere from 3 months to a year depending on the commander. Also considering their performance in the winter war what makes you think the Red Airforce would rule all? If that was the case the Finns would have been dealt with far easier.
I doubt Stalin would invade without extracting every item can out of Germany and would only invade if he could guarantee success and not lose prestige after another phyric victory that once again made the Red Army the laughing stocks of the world.