Once everything belongs to the state, it really belongs to those who rule the state.
Again, not that easy. Khruschev didn’t decide that the iron in the factory #3 would be used in the steel beam factory #7. The planning of the productive forces was an incredibly complex process in which thousands of bureaucrats union members were involved. Calling that amalgam of workers an “owning class”, especially when they’re not extracting surplus value at all from the workers seems a big stretch to me.
Centralism is never democratic.
The fact that the USSR wasn’t as democratic as ideal, doesn’t mean that the existence of a state can’t be democratic. “Centralism” is an umbrella term covering many different possibilities of governance, and a single party ruled by elected leaders of worker councils is a recipe of some sort of centralism that can provide a very reasonable degree of democracy. I’m not arguing this was the case for the USSR. If you want to read on a practical case of the existence of democracy within a Marxist-Leninist single-party regime, I recommend you have a look at a book called “How the worker’s parliaments saved the Cuban revolution”, from Pedro Ross, which describes this exact form of functioning of back and forth between the central government and the worker councils in which millions of Cubans participated to overcome the worst consequences of the “periodo especial” after the illegal and antidemocratic dissolution of the USSR.
I myself am from a country with a rich history of anarchism in the 20th century: Spain. By the 1930s, the CNT, a union of workers which proposed some sort of anarcho-syndicalism (which I bet you’d be happy to agree is a good method of governance), had more than a million members, which for the population of the country at the time was absolutely huge. The lack of centralization of sorts initially among the leftists, and their consequent weakness to respond to threats, is actually the very reason why fascism could trump the democratic government in many places of the country and destroy this anarchist movement and all social progress for the following 40 years. Funnily enough, the dictatorial USSR was the only country which assisted the republicans in their civil war against fascism, other than the admittedly heroic volunteer corps from the brigadas internacionales.