Still struggling here! There are a few venues big enough, but they all seem to be booked up :-(
What about 360cities.net/image/sandomierz-royal-castle#31.10,1.70,81.5
There are plenty of good hotels within walking distance and a beautiful square. I have seen events taking place there.
You could also contact sarp.org.pl
they are responsible for some historical building/Hotels/Palaces known as DOM ARCHITEKTA "SARP" or the Polish Historical society who ca put you in contact with the owners.
Polskie Towarzystwo Historyczne,