Regarding socialist democracy, there are different kinds in different places, from approval voting (similar to what you describe here) to direct competition, but the jist of it is that competition in politics sows division, while cooperation tends to be far more successful. This doesn't inherently have anything to do with democratic centralism, nor does it have to do with Maoism, inherently.

Overall, most of what you've read is good! I support the majority of that list, but outside of Peter Singer, I take most issue with Orwell and Chomsky. Orwell hated the working class, even some of his comrades in Catalonia joked that he belonged on the other side. He was a British fed, and kept lists of suspected Jews, homosexuals, and communists, that he used to narc on people. Chomsky on the other hand is an insufferable sycophant for the US Empire, and fundamentally hasn't made an actual attempt to understand Marxism. Chomsky has some decent contributions like Manufacturing Consent and his work with Ilan Pappé on Palestine, but is a massive anticommunist.

I recommend, if you have the time, reading On Orwell, A Critical Read of Animal Farm, Isaac Asimov on 1984, and On Chomsky if you want to better understand what I'm talking about, here. These are more for providing a counter to them and their work than they are for understanding Marxism-Leninism and actually existing socialism (AES), though, so feel free to skip these.

More importantly, and what you've very helpfully identified (I mean this with no ill-will or superiority, this is a complicated subject and you've read more than most), is the core of your misunderstanding of Marxism (not Marxism-Leninism yet, that's something to discuss later), and how it answers the same questions anarchism tries to in different ways:

Anarchism is primarily about communalization of production. Marxism is primary about collectivization of production.

When I say "communalization," I mean anarchists propose horizontalist, decentralized cells, similar to early humanity's cooperative production but with more interconnection and modern tech. When I say collectivization, I mean the unification of all of humanity into one system, where production and distribution is planned collectively to satisfy the needs of everyone as best as possible.

For anarchists, collectivized society still seems to retain the state, as some anarchists conflate administration with the state as it represents a hierarchy. For Marxists, this focus on communalism creates inter-cell class distinctions, as each cell only truly owns their own means of production, giving rise to class distinctions and thus states in the future.

For Marxists, socialism must have a state, a state can only wither with respect to how far along it has come in collectivizing production and therefore eliminating class. All states are authoritarian, but we cannot get rid of the state without erasing the foundations of the state: class society, and to do so we must collectivize production and distribution globally. Socialist states, where the working class wields its authority against capitalists and fascists, are the means by which this collectivization can actually happen, and are fully in-line with Marx's beliefs. Communism as a stateless, classless, moneyless society is only possible post-socialism.

Democratic Centralism, ie diversity in thought, unity in action, is really the expression of democracy in unified, proletarian form. It goes against individualism and towards collectivism, the biggest strength of the working class is its ability to organize.

Moving onto Lenin, I think you misunderstand the issues Marxists have with "ultraleftism." It isn't the leftism we take issue with, it's the idealism, moving into utopianism and trying to achieve higher stages of socialism when the productive forces aren't suited to it. It would be like attempting capitalism before the steam engine, or feudalism before farming was a thing. Can you imagine being a landlord over a hunter-gatherer society? How can that work? It's the same with ultraleftists, who in Lenin's time rejected working with the unions, rejected the revolutionary role of the peasantry (only seeing the proletariat as genuinely revolutionary), etc.

All of that is leading to this: I highly recommend giving my intro Marxist-Leninist reading guide a look, especially section 0a. 0a describes why we need a collectivized economy, why Marxism-Leninism in particular is crucial, and finishes with my favorite works on dispelling Red Scare mythology and contextualizing AES, particularly the USSR (especially with dispelling horsehoe theory as the nonsense it is). The whole guide is meant to be as highly focused on providing utility to the reader as possible, but that initial section is intended to help anyone come to at least come to an understanding of Marxism and Marxism-Leninism on Marxist terms.

Hope that answers your question!