Why not just adopt Bitcoin itself as a main currency? You could even set up a fractional banking system based on top Bitcoin. The first government to do this is going to have a huge economic advantage for the rest of this century.
There are still way to many liabilities for a government to implement bitcoin as a main currency
There are no cards and if there were you still can't trace payments in anyway like you can with a normal card unless bankers have accounts with their info on it which completely defeats the purpose of a decentralised currency.
If a government adopted Bitcoin, would you still call it decentralised? I don't think so