Playing devils advocate here, Trump has a history of wriggling out of what looked like slam dunk cases against him, and being an ex-president comes with truck loads of political baggage, so the DOJ might just be playing things extra-careful. It would make sense for them to double and triple check everything, dot all their i’s and cross all their t’s, and make sure everything is as utterly airtight as they can make it. The real danger is that they don’t go so slow that the whole thing becomes moot because they took so long that Trump managed to make himself dictator in chief.