In Aug 2017, my wife and I rented a car in Wroclaw, drove to Boleslawiec (and stayed two nights), and then drove to Krakow and dropped the car off. Driving was relatively easy, though the portion into Krakow to drop off the car had the usual stress of driving in a busy unfamiliar city.
We visited Auschwitz during our Krakow stay and chose to take buses as described in the Rick Steves book. That worked very well; we saved ourselves stress by going to the bus station a day early to (1) buy our ticket in advance and (2) walk to the platform to verify where to board the bus for Auschwitz. For the trip back to Krakow we (as described in Rick's book) just caught a bus at the nearby bus stop; it was a smooth process. (Doing this in the cold of December might be a different experience.) (My short trip report on our time in Krakow: https://community.ricksteves.com/travel-forum/trip-reports/poland-wroclaw-boleslawiec-krakow-aug-2017-part-1-of-2)