A bit of a random PoD, but what if we have 1204 go differently?
So a bit more stable Roman Empire (perhaps via a Jewish community effort for the sake of this scenario).
However, the 4th Crusade still goes after Constantinople, only to be defeated in large part due to that Jewish Roman community.
The Romans manage to muster the forces required to retake Venice, and grant the city to the Jews of the Empire, as thanks for their efforts that saved the Emperors city.
The Jews then own Venice, and are in a position where the ERE protects them.
In more detail, an initial PoD where Jews are granted protections after incidents during the First Crusade - a basic law stating that religious discrimination against Jews, Latin and Orthodox Christians. Larger community builds up as a result.
Throw in a Samuel HaNagid figure, major advisor for Alexios III, and a Jewish Merchant Fleet, and you've got the basics of a plausible "Jewish-Roman Venice".
Sure, you've got some butterfly nets in place, but the rise of a strong, largely apolitical Jewish community in the Roman Empire doesn't strike me as something to throw history in the bin.