Can't add funds to Apple ID without using family plan CC

My wife is our family plan organizer. I want to add funds to my Apple ID for in app purchases that do not come from our family plan CC... how can I do this?

When I go to my own account, it will not let me add a payment method. The edit option is greyed out.

When I go to add funds to my Apple ID so that it works, but only takes funds from the family share CC... I can't intervene and use a different card.

Apple, the point of family sharing should be ease of use and not restricting adults to the point of frustration. I appreciate the ability to share apps, but as an adult, I should not have to run my purchases through my wife. Please fix this. My son is about to be an adult, I would like him to be able to have the advantage of sharing purchases with us but also want him to be able to do his own thing with his own money. This should have the options to set the levels of restriction. PLEASE HELP