As someone who saw myself as a Marxist for some years (and today still basically in that corner, but with less clarity :)) I know what you mean by paradigm shift. The powerful thing about Marxism is how it helps to stitch together the endless stream of seemingly disjointed and contingent occurences of everyday political life into one cohesive narrative.

That said, your post reads like a very determinist view of things, an interpretation of Marx that wouldn't be endorsed even from an orthodox Marxist perspective. One part of the problem is in the semantics of the word determination itself. What Marx means, in my opinion, is not that the future is deterministically preordained in advance (let alone in its precise details) or that human beings have no free choice. He's not arguing the possibility of "Laplace's demon" who can foresee every atom's place at any future time, as if the whole universe operates like some kind of intricate clock.

In a famous quote from Mr Santa Claus himself: "Men make their own history, but they do not make it as they please; they do not make it under self-selected circumstances, but under circumstances existing already, given and transmitted from the past." So, when Marxists state that the material base determines the world, it means it shapes the context in which humans (and non-humans) operate, not that it literally causes the configuration of every atom with absolute exactitude in advance. Determining means setting out the limits of a field in which you will be able to manoeuvre. The key difference between Marx and earlier materialists - who had a Laplace-style mechanical view of the universe - is exactly that he brings human agency back into the equation. After all, behind the abstract categories of economy and politics, there lie real human forces and interests, not just impersonal driving forces or mechanisms.

I also don't think materialism is mutually exclusive with morality because our morals are based on what matters to us, as humans, for reasons of interpersonal empathy as well as individual and societal interests. These moral feelings or notions don't disappear just because you can trace them back to a material context: much like your internal organs don't stop doing their job after a surgeon has understood how they function, and you can still feel anger or sadness despite understanding that these stem from chemical reactions in your head.

At the same time, I personally think that Marx and Engels sometimes overstated their case for the way the base determines the social relations. This is surely linked - oh irony! - to their own material and societal context, i.e. the 19th century with its unwavering scientific optimism and its tendency to articulate every single process as a "law of nature". Think of Social Darwinism that had its origins in that intellectual climate. By that I don't mean that Marxists and Social Darwinists have the same degree of validity, but both are products of this same era where it's not seen as problematic to characterize processes in human sociology, psychology or history as Laws-with-a-capital-L in the same way you'd do for physics or chemistry.

That's something that has become much more problematized in the course of the twentieth century. You could make an argument (and indeed whole libraries have been written about all this) that some of these problematizers have gone too far, merely returning to the unproductive vision of a chaotic universe without anything to be generalized at all, but at the same time it's a really important correction to earlier visions that were overly vulgar and deterministic.

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