“Friedrich Engels in a thinker’s pose
The four-meter-tall bronze sculpture of the other philosopher of communism, Friedrich Engels, is a bit smaller than the planned Marx statue in Trier. This Engels monument in his hometown, Wuppertal, was also made by a Chinese artist and offered by the government of China in 2014.”

I grew up under communist rule.
We studied marxism in school.
At some point, I was about 16, the teacher asked us about the relative merits of the different brands of materialism he had mentioned during his classes.
My answer was ‘dialectic materialism is better than all others because those who apply it into practice constantly gouge the consequences of their (political) decisions and fine tune policies accordingly’.
Some 15 years later the communist lager had imploded simply because those who were supposed to act in a dialectic manner had failed to put the principle in practice.
Coming back to the original question, ‘was Marx a determinist’, the answer is yes.
Marx’s dialectics is only a procedure. Meant to help the communists exercise the dictatorship mandated by Marx in the name of the proletariat. And dictatorships are determinist by definition.
Why mandate one if you are not convinced that things can be ruled?
For the long run and in a comprehensive manner?