Which is more preferable after graduation: to work in one's university major or to study a Sharia major, given the family's needs?

There is no contradiction between your work in your field of specialization and studying Islamic sciences. We advise you to balance between them: work in your field to meet your family's needs, and dedicate some of your time to studying Islamic sciences. This is easy to achieve, and its means are numerous. Seek assistance in this by organizing your time and not wasting it, and by supplicating and turning to Allah Almighty, for He is the Granter of success.