Does anyone know a Marxist Critique of Idealistic Philosophies in the 20th century?

I don’t know of any, though I’ve also never looked for them.

Quite a bit of mid-to-late 20th century idealist philosophy came from the Western “post-Marxists” themselves, with the assistance of the CIA. Almost everything I know about it I learned from Gabriel Rockhill, who studied under that school before coming to reject it.

• The CIA & the Frankfurt School’s Anti-Communism
• The Myth of 1968 Thought and the French Intelligentsia: Historical Commodity Fetishism and Ideological Rollback
• Imperialist Propaganda and the Ideology of the Western Left Intelligentsia: From Anticommunism and Identity Politics to Democratic Illusions and Fascism
• The CIA Reads French Theory: On the Intellectual Labor of Dismantling the Cultural Left
• Foucault: The Faux Radical
• Foucault, Anti-Communism & the Global Theory Industry: A Reply to Critics
• Capitalism’s Court Jester: Slavoj Žižek

Not exactly what you asked for but a lot of these "post-marxist" philosophies have at least some of their roots - intentionally or not - in Proudhonisim. Proudhon wrote Philosophy on Poverty after which Marx responded with his critique, the Poverty of Philosophy, which you may find of interest.

Post-Modernism Today - Siraj

It can be a bit dense early on, but it gets better. Can be really enlightening at times, and people who have been to humanities classes will relate.

edit: there also an audiobook on spotify or youtube, if you prefer.