It is meaningful for punching left to refer to antagonizing Socialists. Marxist-Leninists are by far the most common type of Leftist globally, so pretending that they are just a small niche is very western-centric.
As for fascism, you argued that the Soviets should have allowed more opposition. In the USSR, that opposition consisted of Tsarists, fascists, and liberals, all of which ultimately were responsible for killing millions of citizens of the USSR.
As for centralization, you agree with Marx about it centralizing. However, rather than move forward in time, you try to turn the clock backwards. If centralization is a given, then it should be democratized across the whole of society so that we may continue to increase efficiency in production and work fewer and fewer hours to cover more and more needs and wants.
As for how the USSR was run, this is just generally false. The Soviet method of democracy was in place, and the economy was run and planned by many, many, many people. As a consequence, wealth disparity between the richest and the poorest was around 10 times, as opposed to hundreds to thousands in the Tsarist era or the modern Capitalist era. Some "ruling elite" they turned out to be, looks like they sucked at it. For further reading: Soviet Democracy and Is the Red Flag Flying? The Political Economy of the Soviet Union.
In your original comment, you expressed equal distaste for Communists and Nazis. Communists have historically had far fewer skulls under their name than liberal regimes or fascist regimes.
Either way, though, what's your alternative that causes you to break from Marxism? Where is the evidence of its success, and your plan to get there? Genuinely, I am asking honestly.