This is mostly from my research for my TL, so it has a German bias to information, but some of problems/issues/ideas.
1) Malaria is huge issue. Over time, you get drugs to help, but there are ways to minimize. The highlands were seen as better for settlement. Few reasons such as fewer swamps, easier to drain sloped land, temperature closer to what Europeans like, dam rivers for water, dam rivers for energy. Angola, South Africa, East African highlands are consider good areas. Now once you get enough technology, then sure everything works fine. But this is a many decade long problem.
2) Lack of Coal. Oil discovered better late. Lack energy for production outside of South Africa which has coal. Also does not have malaria. So you need hydroelectric, which is seen as expensive.
3) Different food crop package. You can go to South Africa and use Med crops. Not true in much of Africa.
4) Soil. Tropical soil are less fertile due to more rain that has removed critical trace minerals and elements. Easy to fix today with modern science, but in 1880 you don't have the soil testing technology. So take central Brazil as example. Now huge farm area, but it took a generation of work with 1960 technology to first get fertilizer mix right, and second breed/engineer right types of grass and farm crops.
So is it plausible. Yes and no. Yes it is technically plausible but it cost a lot over a 40 year development period. No, no one will fund. But lets use ASB magic on motivation. After Germany unifies the gods of Germany get UK and German to make deal. UK gets many things it wants, but German ask for either Greater East Africa (German East Africa plus British East Africa) or Greater Kamerun (Kamerun + Nigeria OTL) or Greater SW Africa (SWA + Angola + slice of deep Congo basin). German gods decide German people want badly so budget is say 5-10% of size of OTL army budget (Strong Alliance with UK keeps Germany safe). German gods have Germans to move to these areas.
OK, so by 1900 or so, we have major German colony with a few hundred K white Germans. Malaria is under control (manageable). Crops have been figured out. We have rail network. We have good farm/ranching land. We have factories using local materials to make something for export back to Germany. By 1950, we could have white supermajority area that is part of Germany or views self as Germanic.
People see USA taking over so much land, but miss we had fewer issues (less severe strain of Malaria), unpopulated, and we invest a fortune making it work over 100 years or so. Do same investment with Africa, sure you could have it work. Will is lacking. Sure Portugal could have metropolitian Portugal include white Angola. UK could have white Dominion in East African Highlands. USA could have greater Liberia as State. etc, etc.