(1) The Polish-Lithuanian Commonwealth had elected kings and a parliament. It was a republic to the same degree as Rome or some of the more democratic Greek city-states. The only people who could vote were nobles, but nobles were about 10% of the population. That's about on-par with with Athens or the Roman Empire. Even in the US, only white landowners could vote when it was first founded. That's not too dissimilar from the Polish nobility.
(2) Jews emigrated to the Commonwealth from all over Europe for better treatment. Good treatment? No. Better treatment? Without a doubt.
The Commonwealth wasn't a modern enlightened democracy, but this was 1600, not 1900. It did incredibly well for 1600.