If the United States had continued the Good Neighbor Policy and not overthrown Guatemala, Brazil, or Chile and had backed Castro's Cuba, I think Latin America would be much better off today.....
Essentially you are asking for a socialist USA. Which is fine with me! But hardly likely to emerge smoothly out of the New Deal unfortunately. Good Neighbor policy had some benefits but they were mainly short term--if we had continued them, surely Arbenz would not have been overthrown in Guatemala for instance, and hundreds of thousands of Mayans, as well as other large numbers of oppressed Guatemalans would still be alive today.
If Fidel were to grow up facing such a benign El Norte, would he even become a revolutionary at all? Might not Batista be overthrown by more constitutional means, or else he'd be steered into less oppressive forms of rule by observing his American patrons frowning at his antics? But of course his patrons were not American idealists, they were American profit-seeking entities (including criminals like the Mafia as well as legal private firms) and these would have to be actively restrained in their exploitive actions by a responsible US government. A US government lacking sympathy with the interests of bodies like the United Fruit Company, at least to the extent that it demands American investors overseas follow norms they are expected to at home, would behave very differently from the governments we actually had. Before he took the step of nationalizing UFC's plantations, Arbenz in Guatemala levied taxes on them--which the UFC defied payment of. Had UFC acted the same way in the USA of course they probably would not be nationalized, but they probably would be taken to court and forced to pay back taxes and also penalties; this might bankrupt them, and civil and criminal penalties applied to individuals in management responsible for the anti-social actions would shake up their management quite a lot. UFC basically claimed a right to act differently in Guatemala than in the USA, and the US government recognizing the Guatemalan state's right to act as the US government would would be, if not socialist, than at any rate operating on a level of fairness in foreign policy alien to precedent or OTL subsequent priorities.
With the USA acting like that, Cuban development would presumably be rather different. Perhaps the Castro brothers would become radical Marxists nonetheless, and perhaps thus alienate a USA that acts fairly and openly, but still favors private capital, just not overwhelmingly and one-sidedly. Then again even if they were radical, even acting as revolutionary Marxists Castro might be able to strike some kind of deal with the USA. Not so much if he seeks the patronage of a Soviet Union perceived as the Manichean enemy of the USA.
I regard Castro as a legitimate ruler. I've been asked on a number of threads, especially recently, how I dare say that when hundreds of thousands of Cubans violently opposed him--but clearly the fact is far more Cubans supported him, or he'd have been overthrown long long ago. Unlike the satellite "people's republics" of Eastern Europe, there was no way for massive Soviet armies of occupation to daunt and intimidate mass resentment; in any case it has been over 25 years since Soviet aid ceased to assist Castro and despite deep hostility embedded in US policy determined to see him unseated, he lived out his life in nominal power, with real power devolving to his brother and associated junta. That would not have happened if he were not seen by a substantial majority of Cubans as delivering some good.
There is no predicting what exactly would happen in an ATL where the USA behaved differently without gaming it out closely and realistically. I'd think the Castros would probably be more moderate to begin with, and more willing and able to allow standard parliamentary limits on government power to stand, and more willing and able to face being turned out by democratic processes, with more hope that they can come back in again by the same means later. But that would not be true if in fact they were determined to move Cuba toward a communist goal, unless we suppose the USA too is also strongly socialistic and sympathetic to such a goal. Which is a TL with much bigger changes than just a regime change in Cuba!