If Trump had declassified everything like you suggested odds are good he would have declassified potentially damaging things - just so he could take them home.
In this case it doesn't really matter because according to the warrant they're using the Espionage Act which doesn't rely on the documents being classified - only potentially damaging to national security.
I don't think a claim under the Espionage Act that involved unclassified materials would be brought, and if it was, it would be shot down very quickly under the First Amendment. It doesn't sound like the Espionage Act is really on the table because there isn't a claim that he actually did disclose them to anyone else.