When Cuba starts taking IMF loans, that is when I'll get worried.

I would encourage you read about the Special Period. We are Cuba! by Helen Yaffe is good. As rough as things are now, Cuba is still not even close to where things were in the 90s, and the Revolution survived.

It seems like power generation and distribution seems to be the most acute problem on the island. The blockade makes upgrading their power infrastructure especially difficult. But it does seem like Russia and China and starting to maybe help take care of that.

I believe socialism in Cuba will survive (and thrive) because 1.) the people really do have that revolutionary spirit and commitment, and 2.) all you need to do is look at what happened to Russia, Ukraine, East Germany, et al when communism was murdered there. The west made them pay for daring to attempt to do something different. As bad as things are now, they will be much worse if the Revolution capitulates to the US.

Edit: I also want to add, I don’t think the US would lift the blockade even if Cuba adopted free market capitalism. I think if that was the offer, even as a principled Marxist I would have to at least consider that if I was Cuban. There is the orthodox Marxist notion that you need a capitalism phase before socialism, so you could in theory adopt a fully capitalist system and then have the revolution bide its time until the moment comes to retake the country, certainly if the blockade is causing so much pain.

BUT, that will not be enough for the US, so it’s all academic anyway. The US would want political change as well. They can’t change the political system to “democracy” because Cuba already is democratic and objectively moreso than the US. No, in addition to free market capitalism, the US would demand no communist could serve in government, the Cuban government would be staffed with unelected compradors, traitors, and gusanos; or the US would just rule over it like a viceroy. And from there, the US would rip apart the economy and hand the pieces over to gusanos and US corporations so as to “restore their lost property”; up to and including personal residences. Imagine being kicked out of your home and out on the streets because some gusano in Miami had a grandparent who lived there before the revolution. If you think this is an exaggeration, this is literally what happened to East Germany. And that was done to them by a country that ostensibly wanted to unify and heal. The US will have no such illusions about Cuba, ripping apart the carcass and making the Cuban people suffer will be the point.

I think the opposite is more likely. I can't recall any american blockade or sanctions that actually managed to work as advertised. The populace always understands that the US is to blame and becomes more anti-american. Iran, Korea, Venezuela, etc.

Lifting the sanctions would leading to a flood of trade with the US. That would both cause general prosperity, which would decrease discontent, but also give the US far more leverage to threaten to take it away. It would also be a security nightmare because the CIA and gusanos still have a grudge to settle.