Main user (me)… I have two Apple ID’s. One for iCloud (joelfeld@me.com) and the second Apple ID (Joel_feld@icloud.com) used for Media and Purchases.
So the Apple One subscription looks like it needs to be under the iCloud account, not the Apple ID that is used for purchases.
I also have all 5 family members in addition to myself under the Family for a total of 6 people.