Can't Access Previous Purchases under Family Sharing
Finally got around this pesky problem. And actually it was pretty straightforward!

All I had to do was add that old shared account as a separate family member. Hence now everybody only uses their respective iCloud ID for their personal stuff and purchases while sharing their old/new purchases.

This might not be a solution for you since family sharing only accepts 6 family members at the most as of now although there isn't any other solution currently.