Your challenge, if you choose to accept it, is to have Rhodesia survive to the present day. It can be in either its Rhodesia/Nyasaland Federation or it can be its UDI Republic.
My idea: For the Federation, just have it become a full-fledged Commonwealth realm and have it abolish its racist policies, not unlike what South Africa did in the early 1990s.
Perhaps Sithole somehow retains the confidence of his party, and a year later, Chitepo survives the OTL assassination attempt at the hands of the Rhodesian government. When the Internal Settlement comes, most of ZANU consents to join the settlement and contest elections. Seeing all major parties accept it, when Zimbabwe-Rhodesia is proclaimed on June 1, 1979, the world recognizes its existence as a fait accompli, essentially. Mugabe forms a militant breakaway faction that causes some trouble in the years ahead, but you never asked for a utopia.