Your plan is sound however I would tweak it slightly as follows: Kraków to Gdańsk (plane, the airline is LOT and it takes less than an hour); 2 nights in Gdańsk (I would go for a property on the seafront e.g. Marriott or even the Novotel will do nicely, both hotels are a few hundreds yards away from each other) so that you’ve got time not only to enjoy the glorious Baltic coastline away from the chaos of Kraków, Warsaw etc. but to explore the old town as well (truly worth it) and, if you fancy a bit of party atmosphere, Sopot is literally 15/20 minutes walk from either the Marriott or the Novotel (Novotel Marina); Gdańsk to Warsaw (again, LOT Polish Airlines or ~3 hours on the train: if you book first class or pierwsza klasa you will pay a small ‘premium’ but you’ll enjoy 2 free drinks and a decent-ish hot meal, there are normally 3 or even 4 choices on the menu); 2 nights in Warsaw and whatever next to follow.

G