Shall I cancel my iCloud subscription if I’m using family sharing

See the following from Share iCloud+ with your family - Apple Support

When you share iCloud+, any family members using the free 5GB plan are added to your family plan automatically.

If a family member is already paying for their own iCloud+ plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time.