There is a nuance that you did not take into account. North Korea, Venezuela, Cuba are small countries, countries where life is bad are a common thing. Iran cannot be called a small country, but after the overthrow of the secular government, it turned into a virtually medieval country, with a tough, totalitarian government, where a low standard of living is commonplace. This time. Two - such a number of sanctions, with such a variety as in Russia, are not and have not been in the listed countries. They also do not have the collapse of the army, and the loss of confidence in the authorities. There are many more differences. And most importantly, Russia needs to maintain a huge number of security forces, to contain internal tensions, it is necessary to support "Putin's friends" who have lost hundreds of billions of dollars, it is necessary to contain at least 100 million biomass, which, although used to living at the bottom, can start a revolution, to replace an unsuccessful king with another king. Therefore, sanctions for Russia-have a devastating effect, for the countries you listed-an uncomfortable effect.
Iran could have been doing better as well, and technically speaking it has been doing fine compared to places like Cuba or Venezuela, so I think your example is correct that it can't be done to bigger nations.
I have to say Russia is not even a "decent" nation, it is a huge one so trying to do it to them was a bad idea from the start and I get that, I understand that if you want to do sanctions to a huge nation, you are forgetting that the world needs a lot from that nation and this time it was energy, if you do that to any other nation, you would face some other problem but it is a problem nevertheless. That is what I really can't stress enough why the sanctions failed, because world needs energy from Russia.