A rigid subscription to determinism and a rejection of free will implies a poverty of action and a resignation. Anyone can feel free to adopt that, just don't call it compatible with dialectical materialism or Marxist thought more generally.

There are different kinds of determinism that get called materialist, and my argument hinges on separating them. As an example, there is economic determinism (here is someone arguing Marx is not that), which though metaphysically materialist is idealist in the Marxist sense of relying on abstraction that rejects some aspects of causality in the material world. In The German Ideology, for example, he refers to Hegelians as idealist in this special sense because they considered only, to put it crassly, their intellectual circlejerking over Consciousness and so on as though all of humanity was causally downstream from this when that is plainly not the case. Likewise, though it appears more materialist than whatever the Hegelians were doing, economic determinism is still discounting the causality of non-economic factors in the world and therefore meets this particular definition of idealist. Among these non-economic factors, of course, are things like the person's own psychology, or those aspects which cannot be credited to their economic position (we can start with their perception of space if it must be proved that such aspects exist).

So what I'm complaining about are determinist framings that claim the idea of materialism while discounting factors that exist within material reality. Whether you choose to act or not is itself a material factor, and the fact that Laplace's Demon could have predicted it is beside the point. There is no overarching I-Swear-This-Is-Materialist-Guys Destiny that operates independently from your choices, those choices are part of the causal chain as they are both caused and causing. Anyone who uses a phrase like OP of "free will is an illusion" is surely deluding themselves into quietism with a belief in some kind of destiny that is absolutely at odds with sincere materialism. Such people are just renaming Fate to Science and misappropriating scientific anecdotes and rhetoric to clumsily defend this sleight-of-hand.

OP has rejected free will and appealed to a materialist determinism, citing science. This is not exactly a compatibilist framing lol.

OP is being silly, but my point is that a compatibilist framing is one that endorses the idea of free will as an element of a nonetheless-deterministic system, which I think is the only way one can do Marxism coherently. Then again, I suppose this position comes from the fact that I think you need compatibilism to do anything coherently, so this isn't nearly as focused an argument as I thought it was (and I didn't think it was very focused to begin with).

I'm not invested in the philosophical debate itself because it's pretty clear basically nobody actually reads 19th century German philosophy, and rarely carefully,

In my defense, I do read Schopenhauer sometimes, but what you really mean I assume are the more popular authors like the Hegelians and so on. I do make some effort to read Engels carefully, but he has the merit of not being as interested in Hegel as Marx.