Here's the thing...
U.S. society and its politics are screwed up, but I'm not completely convinced it is due to the "U.S. system" (i.e. a congress and an independently elected president). I think any political system is prone to failure if the wrong people have political power.
For the past 4 years, the senate and Trump's cabinet have basically given Trump free reign to abuse political norms. But lets say he was the Prime Minister in a Canadian-style westminster system... Republicans would still probably retain an advantage (since they would appeal to rural ridings, which tend to have more political power). And because in the Westminster system, the executive branch has much stronger ties to the legislative branch, Trump would probably have even more authority to cram through laws than he has in the American system.
Two things he couldn't do are stack the courts and compromise the electoral system.
It's interesting that when you look at any list of the best countries to live in, the top ten is dominated by constitutional monarchies with parliamentary systems. They also seem to be the most stable.