A lot of your criticism rests upon dialectical materialism which as far as I know was a term never once used by Marx. It was used by Engels, I think, after Marx died. Probably when Engels was compiling Marx's notes and unfinished writings. The idea of a dialectical method in Marx's work became popular (as far as I know) due to Lenin claiming that Hegelian logic/dialectics contained "the key" to understanding Marx's underlying method and theory.
So one could criticize dialectical materialism but that's a criticism of later Marxists more than Marx himself (in my opinion.)
My biggest criticisms of Marxism are:
1. Most Marxists take an unscientific attitude towards theory. They develop theory based upon formulas rather than investigation, criticism, evaluation, falsification, empirical methods, etc. Marx himself, aside from the Manifesto, didn't write in terms of simple formulas. His theory was flexible.
2. Historical materialism (Marx's name for the theory) was left unfinished at the time of his death. Later Marxists then divided into at least two camps. The revisionists said that Marx's theory was out of date. The anti-revisionists said that Marx's theory was still correct and needed to be "protected" from revisionism.
3. Some of the unfinished parts of Marxist theory were pretty important. Notably, the socio-political aspect of historical change as well as a theory of class. Marx didn't spend much time on either, despite claiming that history was a history of class struggle. If you read his more developed texts (like Capital) he spends much more time investigating capitalism as a system of relations rather than as a system built and worked by people.
4. The best criticism against Marxism was made by Bakunin. He pointed out that any program which included the use of the state to cement the power of one faction would inevitably turn that faction into a new ruling elite, whether a party or bureaucracy.
>most just trot out the tired meme of not true socialism
Eh… Most officially Communist countries probably weren't socialist in any significant sense. I'd call the USSR "real socialism" in a very limited sense… but even then it's an example I hate to defend.
>By claiming that the relationship between classes is inherently exploitative, they create the justification for aggression against the oppressive 'them'.
Nah, class conflict existed before Marx.