Is there a particular “cure” that was recommended by Marx/Engels for addressing their observations on the relationship between capital and labor in mid-19th century industrialist England that you found specifically impractical for the time?

I think he was referring to https://en.wikipedia.org/wiki/The_Communist_Manifesto Perhaps specifically: "In the last paragraph of the Manifesto, the authors call for a 'forcible overthrow of all existing social conditions', which served as a call for communist revolutions around the world." And we know how those went.

The greatest managers of capitalism are authoritarian communists.