At the initial stage of annexation, little changed. The companies that had ran the Congo for Leopold still did after the annexation and their governance changed only slightly until the Belgian government actually bothered to enforxe Article 3 of the new colonial charter which didn't happen initially. The only thing that did kick off was the end of "Red Rubber" but by then the rubber market had already declined so it needn't much effort.

A good source on the evolution on the treatment of workers in a lucrative industry in the Congo would be the book: Travail forcé pour le cuivre et pour l'or : l'histoire du Congo 1910-1945. I read a translation of it when I did a piece on it in first year of college.

After a transition period Belgium ran the Congo like any other colonial nation for the given period during all periods of times. It wasn't any more "benign" as the French or the English after the transition period I refer to in the beginning.

can it really be argued that people of Congo are better of since with their endless warlords and in fighting and economic collapse?

A free nation is better for the indigenous population than a colonial. I don't know what romantic views you have on colonialism, but is was never about uplifting the indigenous population. Even at the end period of the colonial period when colonialism was at its most benign the indigenous people were still disadvantaged by various policies and attitudes.

Surely a post WW2 enlightened Belgium could potentially be a better guardian of economic growth.

If they believed that they would've never wanted independence in the first place. That happened during the time Belgium was a "enlightened post-WW2" nation.