Okay, I am not going to address all the inaccuracies in your OP about the Soviet Union and their warfare but I'll briefly answer your question.

What happened after the capture of Berlin was actually quite the opposite. The idea of communism was reinforced and strengthened. It seems counter intuitive but the reason for this was the fact that the Soviets saw how much better Berlin and Germany was. People were more wealthy and everything was nicer. But at the same time Nazi Germany, who had everything, decided to invade less fortunate countries, like Russia, for example. The Soviets couldn't understand this, the question was "why do this?" This only reinforced communist and Stalinist propaganda of the "evil" fascists and capitalists who were against internationalist proletariat. They were "exploitators" and imperialists, just like they were always taught under the Marxist ideology.

Also, it is worth noting that after the war the majority of resistance fighters who stood up to the Nazis were not only the Soviets (and Allies of course) but the socialists and communists as well from occupied territories. Those were the people who believed in the internationalism and as a result there was a fair amount of leftist movement in Europe post WWII, it wasn't just "Soviets occupied everything under the gun". Communists prevailed in the struggle and they commanded the morale.

Another point I'd like to emphasize is the fact that there was a new generation fighting in the WWII. We all have heard about the Stalin's purges and the incompetent new leadership. What is not always mentioned is the fact that this new young and incompetent leadership was comprised of the "new" type of men, the Soviet men. Those were ex-peasants who came out of either country or poverty and joined the cities for work and study. Again, this produced a new everyday man who was brought up on the communist ideology, and they believed it! There were hardships but a fair amount of population moved to the cities and the industrialization with 5-year plans were going full force. Let's just say that for the most ex-peasants live got better under communists than ex-Tsarist Russia. I am repeating myself but the children of post-revolution were raised as "good and faithful" communists. And when Nazi Germany invaded, it was the evil fascists attacking the Motherland. When Soviets launched counter offensive, they saw the horrors of the SS. When they moved into Berlin, they were astounded at how much better and wealthier it was but the people were "morally inferior" because of the atrocities and the greed. Communist propaganda and ideology prevailed and reinforced itself on it's own. There were those of course who escaped and left USSR at the first chance they got but it certainly wasn't the majority.

Sources: "Ivan's War" by by Catherine Merridale; Undergrad courses.

So, I think there are three parts to this- the popularity of Communism at the outbreak of the Great Patriotic War (June 1941), the effect of the victory on Communism’s popularity/legitimacy, and then the extent to which there was post-war opposition to Communism- or at least criticism of aspects of the system- by soldiers.

First, it is true that there were lots of difficult times in the USSR pre-WWII, absolutely- particularly the Great Famine in Ukraine, dekulakization (the deportation of peasants deemed too wealthy by the state) that accompanied collectivization of agriculture, and of course, the Great Purges (1936-38). Without detracting from or qualifying how difficult these periods were, I think it is essential to point out that Communism also brought lots of improvements in the decades pre-WWII. Under the Communist government, literacy rates skyrocketed, a great deal of importance was placed on education, women gained considerably more rights and protections than they had experienced under the Tsarist system, and ethnic minority populations also received financial support and many new opportunities. In addition to this, the working class gained access to many new avenues of social advancements and were widely celebrated by the state. So there were reasons that the system continued to have supporters by the time war broke out, even if we can easily imagine reasons why people would NOT have supported the state.

Furthermore, the idea that Communism failed during the war is actually the exact opposite of the conclusion many Soviet citizens seem to have come to at the time. Yes, early on there was unhappiness and disillusionment since the Red Army was unprepared for the attack. But once the German army arrived, and showed that they were willing to kill Soviet civilians in huge numbers, and mistreat others, most people rallied behind the Red Army. This should not be read as full-fledged support of Communism, but a clear choice of supporting the enemy of their new greatest enemy (the Germans- so, a lesser of two evils situation for many). Once the Red Army successfully defeated the Germans on Soviet soil and drove them back westward, people began to see the Red Army as the only thing that could stop Germany, and this in turn began to legitimize the regime again. Amir Weiner has an entire book arguing this (Making Sense of War), in which he says the Red Army victory over the Germans became the new founding myth—or re-founding myth, replacing the events of the Russian Civil War—that everything was built on post-WWII. I do not want to suggest that this is how every Soviet citizen thought, and as people interested in Soviet history, we can also look back and question whether or not Soviet actions constitute a failure or a success during WWII. But many people seem to have bought into the idea that victory over Germany proved Communism had been necessary, and deserved to be in power, as they were the only political movement that could successfully challenge Fascism. (The fact that they had been the loudest critics of Fascism for most of the 1930s also helped, and no one was allowed to mention the Moltov-Ribbentrop Pact.)

Finally, after the war, there were actually some anti-Communist movements that seemed to be on the verge of developing among former soldiers after the war! More precisely, there was widespread discontent, and a popular sentiment among Soviet citizens that in exchange for their intense sacrifices during the war, they were entitled to better living conditions after war ended. At times the state actually unofficially encouraged these rumours, to maintain support during the war. This was true for peasants, who were allowed to believe that the highly unpopular collective farms would probably be broken up afterward ended. Soldiers were also given preferential treatment during the war, and were led to believe that this would continue when fighting ended. However, the Soviet government realized that they had created a huge, armed group that now believed they were entitled to certain treatment, and as the war was ending, shifted their approach to limiting the impact of these soldiers on the population at home. They did this by staggering demobilization (many Red Army conscripts were not released to go home until mid-1946, 1947 or even 1948. They were used to establish Soviet-style regimes in other Eastern European territories.) The state also cracked down on soldiers who questioned the state or criticized it in anyway in their letters written from the front. The author Alexander Solzhenitsyn was sent to the gulag for statements he wrote while a soldier on the front. And in fact, the immediate post-WWII era was the height of gulag incarceration during the Soviet period. Never had more people been placed in camps in the USSR. These people who filled the gulag camps at this time included all the groups that could have challenged the Communists for legitimacy (nationalists from newly-acquired western territories, perceived wealthy peasants from those regions, and soldiers who had expressed challenges to the state or had simply seen too much of the west). Elena Zubkova’s book, Russia After the War discusses this, particularly the potential danger of the demobilized soldiers and the problems for the regime caused by the gap between expectations and reality.

Tl;dr: not everyone was sick of Communism when the Germans invaded, the success of the Red Army against the Germans had a huge, positive impact on the legitimacy of the Communist government, and the the state cracked down on veterans and other groups to prevent an organized anti-Communist movement from breaking out.

Also, in the time it took me to write this, another answer has gone up addressing some of this, so apologies if I'm overlapping with that or re-stating.