E.g. China cannot import some items from EU, although they are not under EU sanctions, but because USA effectively controls EU and prohibits trade of some goods with China.
Same with Russia. E.g. every time Russia tries to sell weapons to some third country, USA attempts to create a pressure to cancel the deal.
I'm sure Cuba suffers from similar restrictions.
Anyway, no matter what you do, an island with almost no mineral resources, no source of energy, will always suffer. It doesn't matter whether the government is capitalist or socialist, what currency you use, whether there's embargo or not.
People living in Cuba will be always poor. It's impossible to change that because it would contradict fundamental laws of the universe: you need resources to grow, you need energy.
It's just that now all of them have more or less the same income. If they'd return to capitalist system, very few will become extremely rich (drug suppliers, brothel owners) and the rest will become extremely poor. Just the way it was before revolution.