Why expand eastward? Sure with Poland as allies your back is covered, but there are several major disadvantages:

1) Russian provinces are dirt-poor.

2) Russia's cavalry is excellent, and yours isn't.

3) The provinces themselves are huge, which will make traversing them with mainly-infantry armies a daunting task.

4) Mongols.

I would much rather wait for either the Poles or the HRE to be excommunicated, preferably the HRE, since again I want to avoid dealing with eastern cavalry. Sailing over to Inverness before the Scots take it is also a good idea.