I think Judaism has always existed to some extent as an ethnicity

The problem here is that the concept of ethnicitiy has existed fpr a much shorter time than Judaism has. When Moses says "let my people go!", "people" is not meant in the same way that somebody who has grown up with nation states and various racisms as culturally pervasive concepts understands the word "people". I'm not saying this to nitpick, i agree that there has always been an idea of Jews being a people, but that did not mean what it means today and we cannot fully understand modern antisemitisms and how Zionism developed as a political strategy against them without taking a look at how ideas like race, ethnicity and nation were framed in the last two centuries, and how that affected how Jews were seen by the socieites they lived in and by themselves. 19th century antisemitism saw in large parts a racialization of Jewishness that was previously imagined largely among religiously anchored narratives. And Zionism specifically took the core political project of 19th century liberalism, the nation state, and tried to turn that into a cornerstone of Jewish liberation. Which in turn meant that it came with a large part of the baggage of European national chauvinism when it was turned into praxis.