The "declassification process" is established entirely within the Executive Branch of the U.S. government. This means the President's actions in handling classified materials isn't subject to interpretation by anyone outside the office of the President.
Classification is a red herring.
the issue under the Presidential Records Act is that these documents belong to the American people, not to the President. Unless they are private papers and the Archivist has agreed that they are, Trump should not have had them.
. Under the PRA, the official records of the President and his staff are owned by the United States, not by the President.
The Archivist is required to take custody of these records when the President leaves office, and to maintain them in a Federal depository.