The last documents re JFK are being withheld because they implicate GHWB in the conspiracy. They also confirm a conspiracy, which was the whole point of withholding them in the first place.
IMO, Trump had access to the last of the documents and he withheld them to recruit GHWB's old WH Attorney Billy Barr to be his AG. Barr was the one who deep-sixed the files implicating GHWB in the JFK assassination, which are the last files to be declassified. Now Barr is apparently willing to suck Trump's ring to keep the truth re the JFK assassination a secret, at least until he dies.