When your grandfather was born, there were three settlements of such name (Ciężkowice) here (on later territories of interwar Poland). I believe that just two of them have importance in your searches.
1) Ciężkowice in the county of Tarnów, province of Lesser Poland (Małopolska). It's a small town at the foot of the Carpathians.
2) Ciężkowice in the county of Jaworzno, province of Silesia (Śląsk). It used to be a separate village but now it is a district of the town of Jaworzno quite close to Krakow.
The basic documentation of evidence of Jews in Poland were vital books kept by rabbis in every seat of Jewish administrative circuite. The vital books contain records of births, marriages and burials. They were kept in two series: original and duplicate. The originals were held in the synagogue archive while the duplicates were sent to the state administrative unite, that is to the proper seat of the local county. So even if the synagogue archive was lost there are still chances that the the books survived in duplicates in the archive of a county.
Nowadays, Jewish vital books are held mainly in to institutions, state archives and registry offices (very rarely but some of them can be found in museums, libraries or scientific institutions). The old books are in state archives while the one which records are still under personal date protection are in registry offices.
1) Ciężkowice in the county of Tarnów, province of Lesser Poland. The town belonged to Jewish administrative circuite in Bobowa.
The old book are very defragmented. They are held in the State Archive in Sanok. They are scanned so you can look them through on-line by yourself.
a) the books of born: 1850-1895, 1903-1909, 1911-1912
b) the books of marriages: 1877-1888, 1890-1891, 1893-1896, 1898-1910
c) the books of burials:1877-1904
The younger books may be in the registry office in Ciężkowice. To get information on them (on the books not the records) one must phone to the office and ask. It's not secret information the clerk has obligation to give you the information.
I didn't make a research but just ran through the available sources and have seen that there are evidence of land and building owners from 1848 as well as rich land registry documentation (going back to 1793), judicial and notarial files and others. The cadastral map must be somewhere too. So it's possible to find the family house or the place where it used to stand (if they owned such), testaments, contracts etc. Such files are very important especially in the causes when the vital books are lost or defragmented etc. There are also photos, among others the photo of the old wooden synagouge (burned down by Germans).
2) Ciężkowice in the county of Jaworzno, province of Silesia (Śląsk). The village belonged to Jewish administrative circuite in Chrzanów.
The old book are very defragmented. They are available just on the spot in the State Archive in Katowice. They are not scanned.
a) the books of born: 1902, 1907
b) the books of marriages: 1883, 1887-1888, 1890-1893, 1901-1902, 1904-1905, 1907-1910, 1912-1916, 1918-1921, 1928-1930
c) the books of burials:1882-1883, 1897-1898, 1900, 1902-1903, 1907, 1909-1911, 1913-1914, 1918, 1920-1921, 1929
The younger books may be in the registry office in Jaworzno. I didn't check the archival sources but might be similar to the previous one.
Some rough guidance of stages of genealogical research in Poland you can also find here https://polishforums.com/genealogy/orphan-refugee-sent-new-zealand-85504/#msg1753360
In any questions contact with me through e-mail: firstname.lastname@example.org