The Black and Baltic seas are already connected through the Vistula-Bug-Muchavka-canal-Prypyat-Dnieper (which was actually one of the very last things the PLC did before collapsing). And the Dnieper-Daugava line is the best natural border in the region, and it is only broken in Belarus in the Smolensk Gate - which is why Belarus is so damn important. Control of the Baltics is secondary because 1) it's not key for fluvial connectivity (though helpful) and the region is rough and kinda isolated, so large scale military ops are naturally harder there on account of the forests and swamps and whatnot. So the region can be kinda left to its own devices as long as it remains neutral towards potential Polish enemies. Latvia counts most (though not that much) out of the Baltics, because they control the Daugava estuary, and Estonia also holds the isle of Osel so it can naturally blockade Latvia if they really wanted to. But again, this is not that big a deal. Lithuania in particular is the LEAST relevant of the Baltics. Their river, the Nemen, is connected to 2 major Belarussian cities (Grodno and Minsk) but those lie away from the main communication artery I described, so even if Lithuania was hostile and blocked connectivity, that would not be that big a deal. They would actually be MORONS not to be friendly too - it'd reck their prospects for economic development.
Belarus is actually a rally interesting place geographically. The country is a kind of river crossroad, with parts connected more to other places outside of Belarus then inside... It's a natural commercial hub for the region.
