But I have to admit, I do wonder whether you actually wrote this yourself.

I can assuage you that all text I post on here was written myself by typing on this keyboard. All stupidity is my own, none of that artificial stuff.

And I honestly want to say, kudos to you for looking into this.

I don’t think getting bogged down in definitions is going to resolve the fundamental disagreement, because I think we’re operating from completely different assumptions about how these systems actually function.

I don't think we necessarily disagree on most fundamental things.

I think Marx’s critique identifies a real structural problem, but I don’t think the historical evidence demonstrates that abolishing private ownership of the means of production is the necessary solution. In practice, the systems that have proven most durable have generally been mixed systems: markets and private property combined with democratic government, regulation, redistribution, social insurance, and public institutions.

I agree that currently all non-failed states contain a component of capitalist mode of production. It seems unavoidable at the current level of development of productive forces and human society to use greed as a (limited) motivator. There are two questions to be answered: which component is primary for each state, and how does imperialism tie in to this.

Now on to where I think we will disagree:

The problem is that the theoretical solution has repeatedly run into the same obstacle: human beings.

The actual problem that most socialist states ran into was not the abstract "human beings" (all systems run into that one). It was imperialist aggression waged by capitalists to prevent a similar organization of workers in their respective states. This, together with internal mismanagement by bureaucrats, are the main contributing factors to the fall of USSR.

The historical record does not demonstrate that attempts to replace capitalism with comprehensive systems of collective ownership have produced the stable, classless societies Marx envisioned. Whatever the theoretical merits of the argument, the practical results have repeatedly been far more complicated.

I think it's important to not underestimate the real achievements of socialist state, for all their failings they did move humanity forwards significantly, even if communism is still far away.

Early USSR was on the forefront of many social-progressive issues, from feminism to LGBT rights to ethnic equality. This was gradually worn down during Stalinism and stagnated in the later stages, but it is still impressive for a state to pick the right answer on so many controversial topics.

Early USSR was also on the forefront of worker's rights and many human rights. USSR was the first country to introduce free universal healthcare. Wealth inequality dropped quickly. It industrialized with immense speed, and was the country that de-facto defeated nazism and fascism. Access to housing and food was made practically a guaranteed right for all.

It had a lot of issues, from the overly punitive prison system to deterioration of social justice and human rights under Stalin to the formation of a "bureaucratic class", etc. All those are things that have been taken into consideration, to various degrees, by most currently existing socialist states.

(also I'd like to note that no currently existing communist party has a classless society as a short-term goal; since ML is a dominant ideology in socialist states, it is believed that a "dictatorship of proletariat" needs to be established, productive forces need to be sufficiently developed, and there need to be mass education and cultural changes before communism is possible)

Meanwhile, some of the world’s most prosperous democracies have developed in the opposite direction: capitalist economies combined with democratic institutions, extensive welfare states, strong labor protections, taxation, and significant regulation.

I don't think those are opposite directions, actually. Today's welfare states were all originally "inspired", or at least significantly developed, by the ruling classes of their corresponding countries fearing a scenario similar to USSR, and placating the workers by providing them with similar benefits. Thus, the two systems developed in parallel directions, and with the fall of USSR you can see a (delayed) deterioration of welfare systems in many capitalist countries, since the fear motivator for the capital is reduced - this is also a parallel "development", just in the opposite direction.

China is an interesting example from the other end of the spectrum, although I wouldn’t describe it simply as “socialism.” China officially describes its system as a socialist market economy, with substantial state and collective ownership alongside extensive private economic activity and legally protected private property.

This is where the concept of "dominant mode of production" comes in. China's economy is centrally planned and managed, most critical industries are tightly controlled by the state, the state owns most big corporations and has significant influence in internal structure of others. Capitalist mode of production is allowed to exist, but it is secondary, thus preventing the worst excesses. This was actually also advocated by Lenin in his NEP program, and is consistent with the ML definition of socialism.

And perhaps the deeper problem with Marx’s theoretical framework is that it assumes we can fundamentally restructure the economic system without having to account for the behavior of the human beings operating it.

Marx does not assume this. In fact he specifically talked about the Base-Superstructure dialectic and "definite forms of social consciousness" (currently this is usually unified with the Superstructure), and this ties closely into the subject of our discussion. Human behavior will indeed need to change in order to achieve communism, however those changes are only likely to come once the base (the economic relations) surpasses the capitalist mode of production. Lenin postulated that to surpass capitalist mode of production, it is necessary to first achieve "dictatorship of proletariat" — a state in which the working class is the ruling class and exerts authority over all others — which is usually called socialism.

Until scarcity itself is substantially reduced, I don’t see a realistic path toward the kind of society Marx envisioned.

Scarcity is not the issue. Existing food and housing scarcity is almost entirely artificial, imposed by imperialists and capitalists to keep a more obedient labor force. There is an issue of greed; however, the capitalist superstructure supercharges the existing natural greed in humans to a horrifying degree.