logistical tether in russia with little to no motorization, non existent roads, incompatible rail lines , partisan action and russian mud. on top of all of that, our front will balloon in size at the same time we must garisson the entirety of the austrian empire + we dont want to look too aggressive or the entente will fuck us up the ass.
moving supplies by sea is an incredibly risky affair and should be avoided at all if possible.
remember as well that water freezes in winter, so we cant necessarily keep up supply year round which would be critical.
also the russians have shown that they dont care for their own cities and are willing to burn them down rather than letting us keep them.
we should probably stop after poland and some security cordon, unless there are spontaneous uprisings in the baltics or Ukraine, but even then, it will be a risky proposition to continue our campaign at that point.