Even with a very early POD?
Even with a very early POD, it's not really possible to make all that territory truly Polish. By the time the Poles emerge as a distinct people, the areas to their east are in fact settled by others. It's not so easy to just conquer all that land, and even less simple to then turn it Polish.
However, even with the 1610 POD, you can get a state that's eventually run by Poland, where Polish culture is the dominant influence, where Polish is considered the prestige language, where the east is settled by Poles to a much greater degree than anything we saw in OTL, and where the non-Polish people of that area have adopted far more elements of Polish culture and language.
That can be done. (Poland literally stretching to the Urals can be done, too, but in the same way that Germany stretching to the Urals could theoretically be done: by killing most or all of the previous inhabitants and resettling the whole area.)
Forgetting about genocide and such horrible stuff for the moment: if you want an earlier POD that makes Polish influence easier to spread, I'd suggest maybe trying to avoid the Great Schism. Is Poles and Russians share full communion within an undivided Church, one
big obstacle to their unification and integration has just been removed.