Trump could literally catalogue all the revelations from the last few months in a single document justifying his case for firing Mueller. And then end the investigation and let the press report on what is in that document.
Maybe that’s why Mueller issued the 12 indictments today. The timing for a firing would be really bad right now.
RE: Timing of the Russian indictments
Yes, that and the fact that Trump is to meet Putin in a few days. So let Trump, with written support of Congress, issue this ruling: the Mueller investigation is complete, no collusion was found, and ongoing prosecutions may continue. Mueller and his 13 angry democrats are dismissed; prosecutions (with emphasis that none are related to Trump campaign) continue with other, regular DOJ staff.