Marx wrote about what he knew, which was the inner dynamics of capitalism as a mode of production. A good start is "Understanding Capital" by Duncan K. Foley (search "understanding capital foley pdf").
Foley concentrates on the economics, is very clear and writes in a modern paradigm cross-referencing contemporary ideas in economics. The book is thematically-organised across Marx's economic work and is short (170 pages).
As a complement, there's Michael Heinrich's "An Introduction to the Three Volumes of Karl Marx's Capital" - this link is to the online PDF or search "marx capital heinrich pdf". It's similarly short (224 pages), well-written and conceptually clear. Heinrich is more focused on the actual contents of each volume. He also writes about topics Marx intended to address but never got around to, including the theory of crises, the state and communism itself.
Marx wrote very little about post-capitalist social organisation. Without data this would have been no more than speculation. It was down to Lenin, Trotsky, Stalin, Mao and many others to theorise later developments - in ways I would argue were profoundly mistaken.