The judge didn't stop the espionage investigation, they granted Trump's request to have a special master review the evidence and remove any personal papers and attorney-client privileged material. The DOJ shouldn't appeal because there's no need as they've already gone through everything. Trump should get anything back that legitimately belongs to him.
His attorney is advancing the bullshit theory that Trump declassified everything before leaving the White House; if that's true, there will be an official record. However, the President doesn't have the authority to declassify nuclear-related material or information that cannot be shared with other countries without the permission of the agency that created the material.
Charges will still be filed after the midterms. This doesn't really delay anything as the DOJ would not file charges before the midterms anyway. They were going to sit on the material until then so why not let a special master review it.
