Only exempt the income if earned. Government checks should be taxed the full amount.
The purpose of a flat tax is to treat everybody the same. When you start differentiating between who gets taxed and who doesn’t get taxed, you end up with the same problems you do now.
You either tax everybody from their first dollar or you exempt everybody from the first 15,000.
But yes, I do agree that social service abuses do need to be addressed.
Including retired military?