This is why, fundamentally, class must be the basis of our analysis.
Do we? The high ideals vs the practical application go to the heart of the problem with tankies vs socialist. To quote Standord Beer, "The purpose of a system is what it does". We can absolutely talk about class as theory, but the end goal of the theory isn't simply logical analysis, its the resulting system that arises from it. I reject the notion that good socialist theory always will lead to authoritarianism. So the problem can't be the theory, but instead the execution.
The more important bit is the liberal conception of free speech and assembly, vs. the Marxist understanding. “Free speech” does not exist. Speech is always dominated by the ruling class of society, in capitalism private media is the dominant force and squeezes out everything in opposition.
I think you're referring to Marx's historical observation was that free speech was restricted to those that could disseminate their voice (because of ownership of capital, press, media broadcast). I don't believe that he was aligning with your assertion that it [can never] exist.
In socialism, capitalist speech is censored. The same applies to association and assembly.
That's also a wrong reading I believe. Marx was talking about restricting speech *as a means to gain power in a regressive system, not as an ongoing means of control of the populous at-large forever.
I would also guess I’m correct in saying you do not support existing socialist states, which you label as “authoritarian.”
Lets remove the ambiguity here. You've named a total of one state in your post in this thread you identify as socialist: Cuba. What other nations exist on your list of described socialist states, which you also contend are not authoritarian?