You are PARTIALLY correct.
EX-/FORMER Presidents can NOT declassify documents - PRESISENTS CAN.
I have posted numerous articles covering the documents, President Trump's actions, and the failed raid.
Those articles spell out the fact that PRESIDENT Trump, before leaving office, declassified not only the documents he legally took with him but also declassified Crossfire Hurricane files / documents.
1st - Yes, basic classification rules allow the PRESIDENT to declassify documents:
Source: The provisions of Executive Order 12958, as amended by Executive Order 13292, can be found at 68 Federal Register 15315 (March 28, 2003).
www.archives.gov
2ndly, 19 months ago the President of the US ordered the declassified Crossfire Hurricane documents ... and the DOJ illegally refused to comply with that order! 19 months later, the DOJ Is not only still criminally refusing to release those documents they are illegally refusing to comply with the FOIA requests for the release of those Crossfire Hurricane documents THAT ARE ALREADY SUPPOSED TO HAVE BEEN RELEASED.
But back to your point- a former President can't declassify documents...
...but PRESIDENTS can....and PRESIDENT Trump did declassify those documents WHILE STILL IN OFFICE.
THE DOJ & FBI KNEW/ KNOWS THIS....MAKING THIS POLITICAL GESTAPO RAID ON TRUMP'S HOME WAS UNWARRANTED (as CNN pointed out), unprecedented, historic, and criminal political abuse of power!