It's very doable - the only question is -- will there be enough time for you do everything you'd like to. Without knowing your interests, it's difficult to comment but we are planning a similiar trip for the fall and, like you, I've spent hours planning an itinerary.
Below, I've jotted down the major (unique) sites, in hopes that this will help your planning, and provided some thoughts. The drives time is from Google but, in the past, I've ended up taking at least 30 mins more than Google estimated-.
Day. Town. Drive Time. Things. To do
2 Rest Day
3 Warsaw Royal Route palaces/Old Town
4 Warsaw History of Polish Jews
5 Warsaw Warsaw Uprising Museum
7 Drive to Torun 2 1/2 hours Old Town
8 Torun Copernicus, Teutonic Knights castle, Cathedral
note - Are you going to Torun to break the drive or because you want to see Torun? We debated this and decided to spend extra time in Gdansk, to simply enjoy Gdansk and visit Westplatte, Stutthof, Oliwa and/or the tri-city area.
9 Drive to Gdansk 1 1/2 hours Old Town
10 Gdansk WW2 Museum
11 Malbork Castle Castle
12 Gdansk Solidarity Center
13 Wroclaw 5 hours to Wroclaw 3 1/2 hours to Poznan W - Old Town/Cathedral Island P=Old Town/Imperial castle
14 P or W W - City Museum/Cathedral P. ??
note - we thought that Wroclaw would have more to offer than Poznan. And - is it simply intended as a stamp-over or are there things you really want too do? Would it be better to spend an added day in Krakow??
15 Drive to Krakow 3 hours from Wroclaw 5 hours to Poznan Old Town
16 Krakow Castle Grounds/Cathedral
17 Krakow Jewish Quarter/Schindler Factory
18 Salt Mine Salt Mine
19 Auschwicz Auschwitz
We will (as you've guessed) be staying in Krakow a bit longer to see --Częstochowa, Nowa Huta, Home Army Museum, Rynek Underground, National Museum, etc.
20 Return to Warsaw
21 Depart Warsaw
Whatever you do, have a wonderful time!!