I don't think I'm moving the goalposts. My claim isn't only "great progress happened in the USSR in terms of material conditions of workers", my claim is "these improvements of material conditions point towards the existence of a more Democratic system than in places where said improvements don't take place". I'm using the improvements as a materialist measure of democracy because these improvements are, in my opinion, evidence of democracy.

It would be a lot easier to defend the USSR if they only intervened to allow the proletariat to hold referendums

Well, obviously, but reality isn't so easy when you're just one socialist country in a world dominated by capitalism. This was very obvious to the Bolsheviks from the beginning, when during the Russian Civil War, 10+ world powers including the USA, Great Britain or Poland invaded them trying to help the Whites and to reestablish tsarist absolutism. The USSR was an example of what is called "Actually Existing Socialism". You cannot get a totally socialist country when capitalist relations still dominate the planet and threaten your very existence, as was the case in the Soviet Union. For example, the main drive of rapid industrialization In 1929 together with rapid land collectivization was not ideological, but geopolitical: threat of invasion. When geopolitical, and not purely ideological reasons, are behind a lot of your decisions, it's hard to do socialism perfectly. I agree we should analyze the problems and failures, but we can also look at the entire experience of the country and, judging by the results, it was very positive and arguably better than any system we've had so far.

As for Molotov-Ribbentrop and the invasion of "Poland": I'm gonna please ask you to actually read my comment and to be open to the historical evidence I bring (using Wikipedia as a source, hopefully not suspect of being tankie-biased), because I believe there is a great mistake in the way contemporary western nations interpret history of WW2 and the interwar period. Thank you for actually making the effort, I know it's a long comment, but please engage with the points I'm making:

The only country who offered to start a collective offensive against the Nazis and to uphold the defense agreement with Czechoslovakia as an alternative to the Munich Betrayal was the USSR. From that Wikipedia article: "The Soviet Union announced its willingness to come to Czechoslovakia's assistance, provided the Red Army would be able to cross Polish and Romanian territory; both countries refused." Poland could have literally been saved from Nazi invasion if France and itself had agreed to start a war together against Nazi Germany, but they didn't want to. By the logic of "invading Poland" being akin to Nazi collaboration, Poland was as imperialist as the Nazis.

As a Spaniard leftist it's so infuriating when the Soviet Union, the ONLY country in 1936 which actively fought fascism in Europe by sending weapons, tanks and aviation to my homeland in the other side of the continent in the Spanish civil war against fascism, is accused of appeasing the fascists. The Soviets weren't dumb, they knew the danger and threat of Nazism and worked for the entire decade of the 1930s under the Litvinov Doctrine of Collective Security to enter mutual defense agreements with England, France and Poland, which all refused because they were convinced that the Nazis would honor their own stated purpose of invading the communists in the East. The Soviets went as far as to offer ONE MILLION troops to France (Archive link against paywall) together with tanks, artillery and aviation in 1939 in exchange for a mutual defense agreement, which the French didn't agree to because of the stated reason. Just from THIS evidence, the Soviets were by far the most antifascist country in Europe throughout the 1930s, you literally won't find any other country doing any remotely similar efforts to fight Nazism. If you do, please provide evidence.

The invasion of "Poland" is also severely misconstrued. The Soviets didn't invade what we think of when we say Poland. They invaded overwhelmingly Ukrainian, Belarusian and Lithuanian lands that Poland had previously invaded in 1919. Poland in 1938, a year before the invasion:

"Polish" territories invaded by the USSR in 1939:

The Soviets invaded famously Polish cities such as Lviv (sixth most populous city in modern Ukraine), Pinsk (important city in western Belarus) and Vilnius (capital of freaking modern Lithuania). They only invaded a small chunk of what you'd consider Poland nowadays, and the rest of lands were actually liberated from Polish occupation and returned to the Ukrainian, Belarusian and Lithuanian socialist republics. Hopefully you understand the importance of giving Ukrainians back their lands and sovereignty?

Additionally, the Soviets didn't invade Poland together with the Nazis, they invaded a bit more than two weeks after the Nazi invasion, at a time when the Polish government had already exiled itself and there was no Polish administration. The meaning of this, is that all lands not occupied by Soviet troops, would have been occupied by Nazis. There was no alternative. Polish troops did not resist Soviet occupation but they did resist Nazi invasion. The Soviet occupation effectively protected millions of Slavic peoples like Poles, Ukrainians and Belarusians from the stated aim of Nazis of genociding the Slavic peoples all the way to the Urals.

All in all, my conclusion is: the Soviets were fully aware of the dangers of Nazism and fought against it earlier than anyone (Spanish civil war), spent the entire 30s pushing for an anti-Nazi mutual defence agreement which was refused by France, England and Poland, tried to honour the existing mutual defense agreement with Czechoslovakia which France rejected and Poland didn't allow (Romania neither but they were fascists so that's a given), and offered to send a million troops to France's border with Germany to destroy Nazism but weren't allowed to do so. The Molotov-Ribbentrop pact was a tool of postponing the war in a period in which the USSR, a very young country with only 10 years of industrialization behind it since the first 5-year plan in 1929, was growing at a 10% GDP per year rate and needed every moment it could get. I can and do criticise decisions such as the invasion of Finland, but ultimately even the western leaders at the time seem to generally agree with my interpretation:

“In those days the Soviet Government had grave reason to fear that they would be left one-on-one to face the Nazi fury. Stalin took measures which no free democracy could regard otherwise than with distaste. Yet I never doubted myself that his cardinal aim had been to hold the German armies off from Russia for as long as might be” (Paraphrased from Churchill’s December 1944 remarks in the House of Commons.)

“It would be unwise to assume Stalin approves of Hitler’s aggression. Probably the Soviet Government has merely sought a delaying tactic, not wanting to be the next victim. They will have a rude awakening, but they think, at least for now, they can keep the wolf from the door” Franklin D. Roosevelt (President of the United States, 1933–1945), from Harold L. Ickes’s diary entries, early September 1939. Ickes’s diaries are published as The Secret Diary of Harold Ickes.

"One must suppose that the Soviet Government, seeing no immediate prospect of real support from outside, decided to make its own arrangements for self‑defence, however unpalatable such an agreement might appear. We in this House cannot be astonished that a government acting solely on grounds of power politics should take that course” Neville Chamberlain House of Commons Statement, August 24, 1939 (one day after pact's signing)

I'd love to hear your thoughts on this