Your entire idea depends on your blockchain based currency being a monopoly. Forcing a fee on spending it will just make people not want to spend or receive your money, and instead will push them to something else.
I guess you are no longer using the USD then?
Monetary policy is irrelevant unless a currency serves as a unit of account. Once a currency is used as a unit of account, you cannot easily avoid using it. Just like you can't buy your groceries with gold.
Bitcoin cannot be both widely used and outside of direct central bank administration.
Gold could escape control (in theory) because there is no blockchain. Bitcoin cannot escape control.
And a central bank capturing 51% will result in either attempt to take that 51% away from the bank,
Did you skip your meds?