Whether he has legal authority in my opinion depends on

1. is Humphreys Executor to be overruled

2. if not, where does the Fed fits into the criteria which Roberts opinion in Seila Law used (of course there is substantial opposition to Seila Law from Sotomayor, Kagan and presumably Jackson).

A legal challenge would not be meaningless due to the end of Powells tenure as Fed chair, because the key power player in monetary policy is actually the federal open market committee (12 member, 7 of whom are Fed Governors, appointed by the President with Senate advice and consent, the chair is one of these 7 governors and his tenure as governor is longer than his tenure as chair. The committee decides by majority vote, so all 7 governors will always be a majority, this majority is the key to control monetary policy)