in 1939 southern Poland was just fields, villages, one or two cities and a few roads here and there.
I realize that Poles think Poland-centered. But to understand try to think just wider - there is now a lot of documents from those times. Without it all your conclusions will be wrong.
You should know that Germans invaded Poland and SU invaded Baltic states and part of Poland not because they agreed to live in piece forever. As you found by yourself there was nothing of special economic interest on all these territories for SU - except their strategic importance for next big war. It was just a part of the game for future big war preparation.
All "big boys" - SU, UK, France, and Germany - understood that they can't avoid real big war and played for time. They used smaller countries as pawns. Czech, Poland, Baltic states and Finland were first victims of this game but they never been an aim for big powers.
Stalin was not ready for big war at the end of 30-s - beg. of 40-s: he just completed "chistka" ="cleansing" of army and communist party to strengthen his dictatorship - he killed tens of thousands officers and left Red Army without head. He also tried to develop heavy industry in order to increase military production. He proposed alliance and military help to UK, France, Germany, Poland and Czech against Germany- but it was rejected and Western countries left Czech for Hitler without war. Poland even took part in Czech dismemberment - showing an example for Stalin. Poland also had a hope for alliance with Germany against Stalin - they couldn’t even think about opposite alliance.
The Western allies plan was to put Hitler for war in East, not West. Stalin just make an opposite plan and succeed in the start - Western allies proclaimed war against Hitler. But they did nothing. Unfortunately for Stalin Hitler decided that SU is weak enough and started the big war in June 1941 - he believed that the war will be short and after that he will have enough resources for war against UK without danger from the East. You all know the end of story and who won that big game in the late 80-s - USA and UK.
It could be disappointing to think about evil targeting you and your countries but found instead that the evil just didn't even take you into account.