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.
If the mid-terms go well for Trump then Sessions will be out if he really is a swamp dweller as it should be possible to get someone else confirmed - assuming the truth all comes out prior to the elections.