1) Japanese government debts are 260% of Japan's GDP.
2) When the bonds the Fed purchased are maturated, either treasury notes or private MBS, the Fed gets paid the principles and due interests. The Fed can buy more with the paid principles or let it run off (reducing its balance sheet). Fed can not keep the interest paid by the bond the Fed had. The Fed takes off the costs (the broker charge),
remittances rest of paid interests to the Treasury Department ( government gets free money to spend).
This is how the QE & QT circulated. Too many US government debts are no good for his reserve currency statute. But it is not such a big deal as other countries like Argentina or Turkey.
PS: Talking about brokerage charges, the Fed, like all other central banks, for instance, PBOC, are for making monetary policies or banking regulations, can not do banking or trading. When The Fed needs to purchase treasury bonds or MBS, the Fed needs a broker. For example, JP Morgan, the Fed places an order of $50 Bil 30 years treasuries to JP Morgan, JPM buys them for the Fed. When the purchases are complete, the Fed holds $50 B treasury bonds in its balance sheet, and the Fed then types in $50 B into the JP Morgan's count in Fed as the JP Morgan's reserve.
Put the link to avoid argument:
Fed Payments to Treasury and Rising Interest Rates