Family Sharing, Separate Payment Methods
The way family sharing is set up, it seems more like the payment method is the owner, not the Apple ID. - There are many instances in the world where ownership is not tied to a payment method. Family sharing should follow these examples, there is no rational excuse not to.
Adults each have their own credit card, and belong to a family. Apple. Fix this.
iTunes gift cards is NOT a fix, and if Apple thinks it is there is really something systemically wrong at the company.