The reason I am telling you that classes are not simply any way we can categorize people but instead have specific relations to ownership of the means of production and distribution is because I am explicitly agreeing with Marx that the history of class society is the history of successive societies of 2 broad classes in conflict, defined by their relationship to ownership of the means of production and distribution.

Plumbers are not a class. Managers are not a class. Both are workers, and share the same relation to ownership of the means of production. Power is not the enemy, the system that divides us based on ownership is.

The rest of your comment is a ridiculous strawman of a party that is unaccountable and that dominates all. Nowhere in socialist history has this happened in practice or in theory. The working class as a whole decides who should be in the party and government. The party is a subset of the working class, with the same social relations to the means of production and distribution. You are arguing against a caricature.

As for revolution, it's the only way to change a state historically.

As for "mixed economies," all economies are "mixed" today. What separates them is their principal aspect, and the class in control of the state.

I propose a system that puts a percentage of plumbers into the halls of government equal to their percentage in the population. same with scientists, and psychologists, and doctors, and electricians, and mechanics, and fry cooks, and teachers, and cashiers, etc.

How do elections work in Vietnam?

What you describe of people from every profession being involved in government is essentially how the Vietnam Fatherland Front works. It's composed of 44 member organizations, primarily unions, a broad cross section of society that anyone can easily be a part of, and they conduct oversight over Vietnam's elections. The actual administration of the government is done by elected officials, who do not have to be members of the Communist Party, or the Fatherland Front. It's similar to what you proposed, except more democratic, since the cross section of society that makes up the Fatherland Front only oversees the elections, while policy is decided through the elections themselves. Even if you claim that the Fatherland Front secretly manipulates the outcomes, the system you're proposing seems to be one where something like the Fatherland Front would rule outright. The only thing missing is the element of randomness, which only makes the people making decisions less accountable and leaves the possibility of someone who's views don't represent what the people want getting into power.

Maybe you could think about it like that: capitalists are the class that, in order to exist, is forced to continuously take capital and invest it to make profit or they are outcompeted by other capitalists. It's not that they are all evil or "corrupted by power", they are not just incentiveized, but forced to act in destructive ways: steal surplus value from workers, increase exploitation above what people can take, chase profits on an exponentially expanding mass of capital in a finite world destroying the planet, cause a crisis about every seven years, cannibalize their own state and infrastructure that they rely on for short term profit, etc.

They inevitably run into these contradictions due to living on a finite planet and having to put individual interests above all else while the modern economies distribution of work and global value chains absolutely require working together.

What you are worried about is real too in a way, but it's not a class. In a socialist state, a managerial part of the working class is not forced by competition to continuously pursue personal gain, but some of them still might become corrupt and do so anyway. At that point, they have betrayed the working class and they might even do so in favor of real capitalists domestic or abroad who haven't yet completely disappeared as a class. It's a possible problem, but other than the class conflict between workers and capital, it's not inevitably the dominating principle of society.

Rather than worrying about a hypothetical future betrayal of fellow workers in thriving and prospering China, we should focus on how many western workers have already often betrayed the rest of the global working class and continue to do so by supporting terrible imperialist wars, mostly in turn for cheap goods at Walmart.