It makes sense that there would be a degree of distance between Conrad and Marlow, especially in light of an outer narrative in which Marlow's first person narrative was nested. That is also a first person narrative, but since it is anonymous, I suggest it can be taken as even closer to Conrad's voice than Marlow's. I notice that the anonymous narrator twice compares Marlow's appearance to the Buddha's. He does this when Marlow begins talking and again just as he finishes and remains silent. This image--that of Marlow discoursing to his shipmates like the Buddha to his disciples--must be important because it is repeated in this quite small outer narrative, and in fact forms a symmetrical frame for Marlow's story about Kurtz. I suggest that Conrad is not only referring to the authority and wisdom that Marlow has gained in Africa, but also (and especially) to the illusory quality of the light on the waves and of pretenses of civilization and cultural superiority. (The Buddha, of course, taught that apparent existence was illusion). This, implies the outer narrator (in effect, Conrad) was the wisdom Marlow learned on the Congo.
I really suspect Conrad is mostly about how "we" are no better than the people we call savages. To show that he has to show the stupidity and brutality of imperialism. But despite (post) modern post colonial hermeneutics, I doubt it was his first priority. I don't think his pessimism was as political as all that.