As noted the idea of things being a paradise if 20 years passes is exaggerated at best : yes, if you imagine your list of favorite things to be done, and then construct a scenario where the Europeans do all of those, then it might have some effectiveness. Might, there is often blowback and they're fundamentally being implemented by people who aren't really interested in the benefit of the vast majority of people over which they rule, and are interested in policies which will benefit themselves the most. Regardless of how good the intentions and policies sound on paper, its amazing how negative they can turn out once you go through a process that self-selects for policies which are beneficial to you first, and where the positives for the people they are designed to "uplift" are a very, very, distant second.
But it gets worse, because European states fundamentally have limited resources and are running colonization on the cheap, so these perfect policies actually don't ever have a chance to be implemented. My main experience is in the French model, where the French very quickly ran up to the limitations of their financial resources in West Africa as the costs of attempting to promote their theoretically progressive assimilation policy after the Second World War (yes, assimilation has its own problems, but it was the progressive view for the time). When bureaucrat wages started being linked to metropolitan salaries, which it had to be if the idea of Greater France as an indivisible republic was to occur, the costs spiraled out of control and the French quickly decided to shift the responsibility for paying these bureaucrats and making laws to the territorial authorities (the later nations of Equatorial and Occidental Africa) which technically expanded their power but effectively gave them a burden and in some territories was designed to attempt to split the independence movements by giving them the choice of keeping unpopular bureaucrat salaries high, or cutting them and harming an important political class. That's just one example, I'm sure you could extend the same thing to education, infrastructure, medicine : they all sound very good on paper, but they are expensive, and European states are not interested in paying the burden associated with that. There's a reason why Dakar's university Cheikh Anta Diop only was inaugurated in 1958 when African powers for taxation and spending started to be expanded, and why the girls school Lycée John F. Kennedy there as well only was founded in 1964. African states spent a lot after their independence, and that was to some extent unsustainable, and they went into debt and crashed in the 1980s, but that had important reasons : there was a lot of infrastructure and development which had to be built. The European colonial powers wouldn't be willing to finance that themselves.
Personally I am very doubtful about the idea of pan-Africanism, there is a reason why most "pan" x states don't work out. Disparate and-already-independent states joining together without a common bureaucracy and with differing needs and objectives is going to inevitably cause tension and they'll split up without much to show for it, such as the Pan-Arab grouping between Syria and Egypt. Even Czechoslovakia, perhaps the best example of two countries being together and doing reasonably well, split up in the end. Of course, there are the national unification movements, but those generally have additional elements at play. There's a reason why pan-Arabism failed, or pan-slavism, or pan-Asianism : the idea of a simple broad racial concept grouping everybody together is not enough to form the cementing of a nation. Nationalism relies much more upon having an idea formed of a nation : thus I would perhaps suggest that the continued unity of L'Afrique Occidentale Française and L'Afrique Orientale Française might be positive and workable : they'll have all of the problems of the individual states I think, but a properly run bureaucracy and a federal government would be enough to keep them together (its amazing what the concept of imagined communities can do, to make states like India stick together) and it would promote regional cooperation and give them somewhat greater bargaining weight in international affairs than if they were individual states. Balkanization of the regions was chosen as a way to outplay local nationalists and to keep French-backed elites in charge, and while I don't think or expect that they would be democratic post-independence, I don't think that their government problems will be any worse than individual regions. And it would lead to more equitable development throughout the region, increased trade, and possibly more efficient development plans. By keeping together fiscal and monetary policy too, instead of the Franc CFA and independent states, it might also solve some of the economic difficulties.
The competing policy, division on ethnic lines, is one which in the majority of circumstances is impossible in my opinion. Beyond the fact that there are a lot of ethnic groups, and a lot of these ethnic groups aren't particularly well positioned to have a functioning nation, its also one which is going to cause inevitably a lot of chaos. Being able to redraw the lines is going to mean that countries are going to face civil wars, fighting, and ethnic cleansing, because it is functionally impossible to really draw a line clearly. People are inevitably mixed on both sides of the line, and a lot of people don't really have a solidly defined ethnicity in the way we would think of it in a pre-nationalized setting : in Austria-Hungary when you asked people about their ethnicity, often times they didn't know, or they would name one which we wouldn't recognize as an ethnicity today. It is better to attempt to focus on building up civic nations and civic nationalisms which don't leave ethnicity as a dividing line like that, and in my opinion bigger states are one which would function better for that, since they wouldn't potentially leave one ethnic group in charge and risk marginalizing others, nor leave them across national borders.
But really in any scenario it takes time for the African states to deal with their problems, because they're dealt a bad starting hand and they have to take time to work on it. It took a long time for Asian states, for Latin American states, for European states, and it takes a long time for African ones too.