When was the last time you visited cuba / talked to their people directly?

I went around 2018ish, and your take doesn't track. You're right, Russia doesn't have the heft to support Cuba -- but they haven't had the heft to support Cuba for decades. Most Cubans in their 40s that I met, looked back at the days of a stronger Russia as an ideal time period, where they had more goods available -- they literally long for the good old days when Communism was bigger on the world stage. They clearly associate/blame the USA's actions for causing unnecessary famines and issues. Why on earth would you think people would be favorable to a foreign nation that is literally enacting economic warfare to starve the population?? "Thank you for the boot, may I have another?" What??

One of Cuba's main 'exports', are doctors/medical staff for disasters facing other countries. Becoming a doctor in Cuba is basically free of charge, but you gotta work for the govt for a few years after you graduate -- as a low paid, outsourced worker as part of these relief teams. Cuba's govt charges western-ish rates for assistance, but pays those docs a lot less, pocketing the margin for funding govt activities / programs, such as their medical programs. Practically any country that has had a major natural disaster, has gotten assistance from Cuba in this way, because of how they've structured the program -- so I reckon most of those countries would be friendly to Cuba. "Big" western nations aren't the only 'friends' Cuba can have. Heck, there've been articles from numerous news sources even just recently, noting that increased American aggression on this front is impacting health services in various other countries.

America has a lopsided, bully-oriented, military approach at present. They spend an absolutely absurd amount on their military -- hell, they could probably blow up most of the UKs government if they wanted to, which is why your politicians are being such pussies when it comes to the US going rogue and openly boasting about committing war crimes. They have the operational capacity to attack/destroy most other governments. It's not clear they have the capability to occupy and control additional land / regions though, as evidenced by their fumblings in the middle east in recent past, and in that they didn't stick around in Venezuela (they didn't even topple that regime, just abducted the dictator at the top). Having soft power / the support of the people, is an essential component to that sort of thing -- and seeing as America's recent actions are pissing off / alienating even Western powers, I see no reason why anyone would think they'd have much 'success' in occupying foreign lands. They'd meet active resistance, just like their attempts to setup in Iraq/Afghanistan -- likely to a much larger degree even.

You are vastly underestimating cuban peoples commitment to the revolution (they on avg have better QOL than an avg american). Calling it regime you are manufacturing consent for invasion but again you are western british chauvinist so this is expected.

If the US went in with the big stick, Cuba would fall in a week

The US has been in with the big stick for 50+ years against Cuba and they've resisted not just fine but with a higher life expectancy than the US. They just haven't invaded militarily so far because they expect greater internal and international support for the Cuban government as a result, so their strategy is to enforce hunger through sanctions. From the US office of the historian:

"Salient considerations respecting the life of the present Government of Cuba are:

1. The majority of Cubans support Castro (the lowest estimate I have seen is 50 percent).

2. There is no effective political opposition.

3. Fidel Castro and other members of the Cuban Government espouse or condone communist influence.

4. Communist influence is pervading the Government and the body politic at an amazingly fast rate.

5. Militant opposition to Castro from without Cuba would only serve his and the communist cause.

6. The only foreseeable means of alienating internal support is through disenchantment and disaffection based on economic dissatisfaction and hardship.

If the above are accepted or cannot be successfully countered, it follows that every possible means should be undertaken promptly to weaken the economic life of Cuba. If such a policy is adopted, it should be the result of a positive decision which would call forth a line of action which, while as adroit and inconspicuous as possible, makes the greatest inroads in denying money and supplies to Cuba, to decrease monetary and real wages, to bring about hunger, desperation and overthrow of government."

Most Cubans have deep connections with the US and they know the different in quality of life

And most Cubans are aware that the reason is the economic embargo, and that at least they can afford rent and healthcare unlike yanks.

There’s be very little support from the population towards the regime

You're absolutely making that up.

I honestly don't know why they went Iran instead of Cuba if they wanted to wave their big dick around.