In that case. How about some sort of PoD, probably in the early '60's (before the situation becomes too radicalized), where the UK solves the Rhodesia problem with some sort of deal (likely involving Nkomo) where the whites feel secure (thus keeping their capital and skills in country for at least 1 more generation) and the native population has political power and opportunity?
Whoever strikes that deal probably deserves a dozen or so Nobel Prizes of course (I have zero clues as to how to strike that balance), but if you could manage that, it would likely set an example for Angola and Mozambique to follow in the early 70's. With a vibrant ag sector shipping through Mozambique, Zimbabwe is competitively positioned to reach markets in South East Asia and the Middle East.
One thought I had on how to equitably resolve the issue of whites owning ~90% of Rhodesia's farmland during this hypothetical "velvet decolonization" would be swapping ownership stakes in new Government sponsored industries (almost surely financed by American or British loans/guarantees) for land. Say this made ~40% of farmland available to black ownership (obviously imperfect, but compromises seldom are) and developed industries that refined the ag commodities into value added products (beer instead of barley, cloth/clothing instead of cotton, etc,etc). I have no idea if this could work as my Zimbabwe knowledge is limited to a book by Peter Godwin and a couple of hours on google/this site
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Really, production ag is full of virtuous cycles once it gets rolling, it just really needs that political stability to ever get past subsistence for survival in the first place.