Try this idea Poland was actually a fairly powerful and posperous kingdom. It was an historical accident that the nobility became massively undisciplined and paralysed the government. The reason Catharine and Frederick, along with Maria who "wept, but took," destroyed Poland is that the Poles were getting their act together.
Either keep the
Piast Dynasty active after 1370 or prevent Jesuit influence in Poland (the Jesuits worked to destroy the civic culture of Poland because they believed that only such destruction would keep Poland Catholic). In any case, keep the Polish noble civic minded and Poland creates a far more stable and liberal (in the sense of respect for individual liberties) Eastern Europe.
Another path to Liberalism and constitutionalism in Eastern Europe might involve the corenation of
Czarina Anne . There was a move by the nobles to force a constitution on Anne. At the time Anne was in a weak enough position to have had to except an constitution. But infighting amoung the nobles cancelled the idea. In a world were Anne, and after her Elizabeth and Catharine, needed to except a constitution, Russia might have evolved into a parliamentary democracy.