The president can't declassify something without communicating the fact to someone nor can he declassify something without defining what that something actually is. The whole purpose of classification is make known what has limited access and who can access it.
Believing that president can generically declassify hundreds of documents and fail to communicate the fact is absolutely preposterous. No federal court would go along with Trump's claims.
However what is likely is the court and a jury would see his actions as self-serving at the expense of national security. If the president unclassified hundreds of documents, including the most sensitive information simply because they were at Mar A Lago then all that information would becomes available to the general public which includes enemies of the United States.
Wrong.
What you are referring to is if the president wants to have documents status changed so that they can be open to the public.
That is not relevant to what we are discussing here, which is the fact all documents presented to a president become automatically declassified to that president, and anyone he decides to use in their maintenance.
It makes no sense for any president to go through the motions of declassifying documents so that they can be read by the public, is they are trivial, obsolete, duplicate, etc.
So clearly what Trump likely was doing was sorting them.
They needed to be sorted in order to determine what to do with them.
And regardless of what status they may have had at one time, they have only the status that the former president decides they should have after no longer in office.
They are not his personal property, but are totally and completely under his complete discretion.
The only complaint anyone can really have is that he was lazy and slow to get around to sorting these documents.
If there was any legal infraction, it was that most of them should likely have been put in the National Archives, to satisfy the FOIA. But the reality is that all politicians likely sanitize anything before giving it over to NARA.