If a navigable Congo linked Germany's African colonies

What if the Congo had no rapids and Germany's African colonies could communicate directly?

Under that assumption, stronger communication would make the campaigns harder to finish quickly and require more opposing force.

The scenario assumes that removing the Congo's rapids allows direct communication between the German colonies associated with modern Namibia and Tanzania. Better connection would make it harder for their opponents to finish the African campaigns quickly. More force would be needed to achieve the same objectives. This is a consequence of the route assumed in the question, rather than a claim that removing rapids alone would create that connection.