First, and probably most important, is "separation of powers".
It has long been recognized that our Republic consists of three co-equal branches; the legislative, the judicial, and the executive. Each is protected in its duties from the other two. None of them has the authority to surrender their powers to the others.
The other term missing from the thread is "executive privilege". This term describes the fact that a President cannot operate in a vacuum. He needs advisers to inform him and make recommendations to him. The President is not required to provide details of important deliberations to anybody.
There are "checks and balances" in the system designed to prevent abuse of office. The authority of any branch of our government is not unlimited. The basic limits are spelled out in the Constitution and others are defined by the various precedential legal cases.
The bottom line in this case would appear to involve whether Congress, in writing laws regarding classification of documents, has the authority to limit the actions of a President. The President is the caretaker of the nation's important secrets. Presumably the President is privy to any secrets that might impact the nation's well-being. As President, he requires the authority to use the information he has, whether classified or not, to carry out his duties.
Just as the President does not have the authority to dictate to Congress how its committees conduct their business, Congress does not have the authority to dictate to the President how he conducts his business.
I would suggest that it is virtually impossible for Congress to limit what President Trump did with the documents in question. It would be a big mistake for Trump to declare that he carried out any particular process with regard to those documents. It is sufficient that nobody else can claim that he committed a crime in his handling of such documents.
As for the idea that Biden "re-classified" the documents, the burden on Biden of such a claim would be great indeed. How does one classify documents knowing that copies exist outside the government?
So if he wants to argue that he did declassified those documents while he was still President, his lawyers have to submit some shred of evidence to support that. At least an affidavit or declaration from Trump himself stating that he declassified them. But they won't do that.
One thing that is important to remember here is that if President Trump did declassify those documents, then they are declassified for everyone as of that instance. Anyone could make them public without repercussions.