There were 7 major political parties in Germany in 1933, so I’m unsure that there is overwhelming evidence that more than 2 political parties is protective against extremism.
There wasn't 7 major parties. Five maximum, even two could be argued. But '33 Germany is a weak argument against multiparty systems. Interwar Germany was not a well functioning democracy at all. They had armed street fights and deep political chaos going on for over two decades at that point. Hitler didn't have the majority and formed a coalition government. Only because Hindenburg agreed to dissolve the Reichstag could the nazis take power fully.
So the number of parties did actually block Hitler, and Presidential powers to subvert democracy was the problem. In modern multi party democracies an inability to form a government will result in a new election, not installing a dictator.
1933 Germany was already a failed state, you shouldn't infer anything from that.