I want to family share, but I want all members to pay for their own purchases

The purchases really are separate, and are forever linked to the Apple ID that purchased them. Family Sharing addresses approvals, payment method, and app sharing. You can configure Family Sharing so that family members lacking the "Ask to Buy" option (minor children, usually) need to obtain the Organizer's permission, or approvers designated by the Organizer, before buying anything. The idea is for you to designate your own method of reimbursement, keeping Apple out of it.

Once a member obtains his or her own payment method they can remain Family Sharing members, but there is no longer any significant advantage to it.