1935 is, from a technical POV, almost no challenge at all. Remains the problem of convincing Brussels in time that Congo needs black teachers, engineers, officers, cadres, etc. Training them and letting them gain experience and gain promotions will take time however. On the bright side, the infrastructure part existed in OTL, at independence Congo had a well-equipped port, good railways etc. 40 years later, they had the remnants of the same
So, have Belgium after WWII decide to integrate the Congo in a Belgian Union of sorts as an autonomous state, somewhat like France or Portugal did. This would mean a even larger 10-year plan than OTL, with the first university opening in ~1950 rather than 1954, and with more secondary schools being built. Slowly start diminishing segregation and open up lower officer ranks to Congolese who graduated from the Royal Military School.
Basically, have the Congolese become partners in the whole undertaking, rather than just workers. This should pre-empt some of the troubles of the fifties and allow a gradual, prepared independence in the mid-sixties or so rather than the rushed affair in 1960.
This way, at independence, there are some Congolese engineers & officers, with rising numbers & the government members will have gained experience in preparing the independence together with the colonial administration.
Ideally, Belgium would keep some troops stationed there, and continue to invest heavily in the country, only without OTL's patronising attitude which caused the Congolese to chase us away shortly after independence.
Unfortunately, I still don't really see how to give Brussels a long-term vision, Belgium is notoriously bad at long-term planning (read, we usually don't have any at all), let alone the long-term vision needed to implement such a development plan.