The Treasury and Federal bank act, but the parameters - budget, operating conditions generally etc. are set by congressional mandate. So the Treasury can print money, but if they go off printing $10,000 bills and mailing a wad to every taxpayer, that would probably violate some terms of the laws they operate under.
The president has certain powers of decree in emergencies, but there are serious liimits and generally he can’t do an end run around congress.
One of the fundamental concepts of justice in the USA is that the laws have to be passed by congress and must be clear as to what is not permitted. Passing too broad a law “The president can make illegal any activity he chooses” would likely (a) violate the “too broad” proviso, and (b) since the law itself does not specify the crimes, technically it is not valid for the president to make up crimes later.
Of course, this relies on SCOTUS having the necessary strength to stand up to the executive. Even if they did not, given a ruling from the top court that they were illegally imprisoned, there would be a mess of people sitting in jail waiting for the next change of government to seek revenge on the entire apparatus that kept them there. I wonder if after the coup, “only following orders” would work in that situation, when the warden, police chief, and head of the FBI are hauled up for violating the civil rights of thousands, contempt of court, etc.?