I suspect some if not all of the documents the government wanted back had to do with fake Russian collusion matter. He knows and we know once Trump is out of WDC and the government gets possession of those documents they will disappear.
You are absolutely correct, however, it goes further than that. Some of the documents were kept so that he could refute claims, such as Milley’s, when they claim that Trump did such and such and they stopped it. Trump knows that many of his former officials/hires are going to write books, make statements on tv or write articles concerning Trump which are designed to interfere with his campaign.