The Fed is subordinate to the executive, even if it thinks it isn't.
If the executive is bankrupt they might use the trillion dollar coin loophole or they might just actually take control of the Fed.
That would be just about the fastest way you could spike US Treasury yields into a debt crisis.
And let's not forget that the US is heavy into short term debt that needs to renew: "As of Q3 FY2026, approximately 33% of U.S. publicly held marketable debt is due to mature and require renewal (refinancing) within 12 months."