There’s only going to be one shot at draining the swamp sufficiently to have a major impact. This means they need to get all the significant players in the first go or it will fill right back up to where it was or greater in order to protect itself against any future attempts.
The earliest this is likely to begin happening is a few weeks or a month before the mid-terms in order to get things started. The first wave could be Trump declassifying all the documents and getting them made public so all can see what really happened. After the election would be the 2nd phase and would be all the firings and/or prosecutions.
Your scenario is probably correct. I believe Trump has more than sufficient evidence of corruption in the FBI and DOJ by now to go before the American people and make his case for cleaning house and reforming these rogue agencies. There is literally a mountain of evidence that he can use to justify his actions.
I think the American people see the need for, and want to restore integrity back into these agencies. They will support him on this. I agree with your idea that he should start with declassifying the documents. This will give the public some time to digest the depth of corruption and expose the abuse of power by people who violated the trust placed in them.
For the sake of the country, the cabal of partisans that have infested these agencies has to be cleaned out. And soon.