We know there was some discussion of the 25th Amendment at the end of Trump's term because of what happened at the Capitol. Now Michael Isikoff of Yahoo News
posts (I have expanded abbreviations in the quote) some news from three years earlier:
While at the White House, Steve Bannon concluded that Donald Trump was suffering from 'early stage dementia' and later started a covert campaign for invoking the 25th Amendment to remove him, veteran 60 Minutes producer Ira Rosen tells Daniel Klaidman and me in a new Skullduggery podcast. (He's got Bannon's texts to back it up.)
As I recall, one of the first books published about the Trump administration, Michael Wolf's
Fire and Fury, which appeared in January 2018, was understandably dismissed by many for focusing on frivolous matters and was generally believed to rely heavily on the claims of a few axe-grinding Trump staffers, Bannon among them, did include a passage in which Bannon was said to have warned Trump that he was more at risk of removal via the 25th amendment than via impeachment. Pretty sure nothing was said in Wolf's book about Bannon himself trying to engineer that particular outcome.