If the nearby Communist states and China were willing to fund, train and arm guerillas to attack Rhodesia, why would they stop providing support to Zimbabwe once it had gained its independence?
For example, during the land seizures, a lot of skilled expertise in managing the economy was lost, however, wouldn't Communist-aligned states be willing/able to provide materiel and educational support to replace the lost skills in agriculture? It seems like once Zimbabwe had gained independence, it was abandoned by the Eastern bloc to its own devices.
You mention land seizures, but those didn't happen until the late 1990s, almost 20 years after independence.
I don't think it is really accurate to say "the Zimbabwean economy crashed shortly after independence". From 1980 to 1982 the economy grew strongly, followed by an equal period of contraction from 1983-85. Then less spectacular growth until 1990. Then contraction again until 1993. Even more sluggish growth until 1997, and then sustained contraction, and economic freefall from 2000-2008.
Now, white farmers did leave Zimbabwe in the 1980s. But that was not due to land seizures, the ZANU government was still committed to the policy of "willing buyer, willing seller" in the 1980s and 1990s, so these farmers were reacting to the fear of political and economic disenfranchisement, rather than any real policy program. But, a large number of white farmers remained in the country until 1998-2000 when the land seizures really began.
u/Profrhodes talks about the post-independence economy in this episode of the podcast.
As for why Zimbabwe's economy contracted from 1983-85 rather than continuing to expand, it is useful to compare GDP to it's neighbors. here is Zimbabwe compared to Zambia, as you can see, both countries see growth and contraction that are synchronized with each other. Here's Zimbabwe and South Africa. You can see that same sharp contraction from 1981-1983 and recovery, although South Africa's economy doesn't see the later periods of contraction in the 1990s and 2000s.
My guess as to the cause of this contraction in 1982-85 is the commodities price collapse after 1980. Zimbabwe, Zambia and South Africa were comparable in the 1980s as relatively industrialized African economies, which all had substantial mining sectors producing copper, gold, zinc and nickel for the global economy.
After the 1979-80 oil price shock (where oil prices spiked sharply) there was the "oil glut" where production increased and oil prices fell steadily from 1981-85. At the same time, across the global economy, prices for all commodities broadly declined, and many, many export-oriented commodity producing economies in the Third World saw economic contraction in this period. Here's Nigeria's GDP for the same period where you can see the sharp decline in oil prices leading to a huge contraction in the Nigerian economy, because Nigeria was a major oil producer.
So, Zimbabwe was pretty similar to many other African commodity-producing economies as it ran into trouble in the early 1980s adjusting to the decline in commodity prices.