The biggest impediment to white settlement in Africa was the tsetse fly. The second biggest impediment was the sheer loss of life from the two world wars, which reduced the population of young people in Europe. And I suppose the third impediment postwar would have been the spread of birth control technology in Europe.
The tsetse fly held back white settlement well into the 19th Century. Coupled with Africa's landforms, which meant waterfalls and rapids at it's continental margins, the fly killed draft animals and meant that all travel would be on foot until someone could construct a railroad. And that agriculture had to be either from orchards or by digging stick or hoe. The agriculture did not change, except in tsetse fly free zones, until tractors and harvesting machines came in, first using steam and then, internal combustion energy by the 1920s.
By then, the two major colonial powers in Africa, Great Britain and France had lost much of their young people to the war and had few young people inclined to immigrate, few young people with the need to immigrate from the UK to the Dominions (and for those who did, Canada, Australia and New Zealand were much more congenial and for the French Algeria) and the Germans had lost all of their colonies. Belgium and Portugal simply had too small populations to generate that many potential migrants and Italy, too small colonies. World War II made the situation even worse, especially when the birthrate fell off.
Decolonization was simply a moralization of a necessity; preventing colonists of differing race and ethnicity from migrating to the mother country and overwhelming the static or declining populations there. Decolonization made it possible to transition to American style imperialism in which rule or influence is indirect, through local elites.
If one WOULD have a large white (or white/mulatto) population in Angola and Mozambique, and possibly much more of Africa, this will require a POD somewhere around 1808, after the Portuguese Crown flees to Brazil. The King must decide that Portugal, here in Brazil, will devote resources to building a large navy. Brazil certainly has the wood for it; at Belem if nowhere else. Then, using these ships, as Portugal is reconquered from Napoleon, displaced people are transported to Brazil as bound servants to work off the cost of their passage, rather than left to starve in Portugal.
This leaves Brazil with a lot more people 10 years later and for the King to be able to resist pressure to return to Lisbon. Portugal and Brazil stay together as an anomaly, a part of Europe governed from the New World, much to Great Britain's dismay. And by 1825, that anomaly has opened Angola and Mozambique to white settlement from a much larger population base. --and after sending an explorer from one to the other, disproving the theory that the interior of Africa is a desert.