One of the key reasons that a country like the USA can enforce sanctions is that it can tell other countries that if their companies/citizens/etc break the sanctions, then the USA will put restrictions on those countries ability to trade with the USA, which is why only a few countries will trade with Iran, North Korea, and Russia

At some point those countries will also tire of the USA trying to tell them what to do and will tell the USA politely to fuck off. This is basically what India did with US sanctions on Russia and Iran.

Given the USA has completely torched its reputation amongst allies in the space of a year, I would expect this to happen more often.

Exactly right. The term I use is "economic network effect".

Does anyone know why does the US allows Canada to trade with Cuba? That one always stood out to me as a bit odd. My guess: Canada is the largest trade partner with the US, so the US probably has more to lose by sanctioning Canada.