I had a deployment with Polish soldiers. They are a modern and extremely capable defensive force. It would likely turn into the same thing we're seeing in Ukraine (smaller force that is well trained and better supplied vs sending a lot of bodies and old tech). I don't think we'll ever see a Polish led force invading Belarus or Russia without a massive NATO backing and a lot of foreign units already on the way to support them. Defensively, they will fuck you up. Offensively, they don't have the man power to go deep into enemy territory.

The Polish have historical grudges with Russia and are chomping at the bit to hurt some Russians. It is probably NATO that is holding them back from sending units to Ukraine