This guy has made a grave mistake with the route planning and the general information Chris. First of all, he should have planned his routes with Komoot in the "Cycling" mode. It would take him by zero traffic roads, picturesque terrain; he would have avoided the construction works or the flooded road, and Komoot would also tell him cycling was not allowed in the Tatra National Park.
Secondly, Komoot offers plethora of fantastic ready routes for Cycling, Road Cycling, Gravel Cycling and MTB.
You are right about taking the trains. He could've travelled from Gdańsk through Pomerania, Varmia, Mazuria and Sudovia (the Lakeland) then take a train from Suwałki right to Warsaw. Then, he could take a train to, say, Częstochowa and cycle through the Jura(ssic) to Cracow. I could find many attractive scenarios for him. There is an Intercity Premium from Warsaw to Wrocław. As
@DiggyGun already knows, Lower Silesia is the most beautiful montane region of Poland, perfect for mountain cycling.
Yes, ask me and I will plan a beautiful stay in Poland for you, the weather and season permitting.