Another strategy point. I have seen the question asked elsewhere, what took so long?
One of the specific statutes they are charging him under is 18 USC 1512(c)(2), an Enron era law passed in response to Arthur Anderson's destruction of documents as the government was investigating. It is the "corruptly obstruct an official proceeding" charge. This law has been used repeatedly and successfully in prosecuting the Jan. 6 rioters. It has withstood every defense attempt to say it does not apply.
Establishing that history of precedence took time. I imagine that has something to do with the delay. Demonstrate success, and a lot of it, on the small fishes before going after the big fish.