My thoughts are - IF Barr/Durham is considering a conspiracy or RICO prosecution then they have to include a good case for all of those being charged. If, say, 120 people are included in the case {DS bureaucrats, politicians + media derps + whoever} then it takes time to legally establish all the connections.
That is my understanding. I suppose DoJ could refile the conspiracy or RICO case to add new defendants, I am not sure. I am just saying that I suspect that if they are filing a broad based case it might take time to establish the total scope of the conspiracy / RICO.
Of course it is possible that DoJ is just dragging their feet in hopes that DJT loses the presidency either through election or impeachment. I DO know that AG Barr has said that if they do not complete their case prior to November then there is a chance that the whole effort will be for naught.
I am 100% sure the whole shebang has taken this long due to the RICO prosecution as you describe. Plus that foreign actors were involved is more legal complexity.
I am also postiive that the DoJ is not dragging their feet. Sessions cleaned it out mightily, Whitaker continued cleaning, and Barr is a honey badger on the side of law and justice.