The 25th covers incapacity.
which is not the same as unfit.
It's been used temporarily for presidents undergoing anaesthesia.
It's clearly designed mainly for such situations, although it was brought in after the Kennedy assassination when the theoretical question was posed "what if he hadn't died and had remained in a vegetative state?" There would have been no way to remove him.
President Woodrow Wilson had a stroke in 1919 and was left badly incapacitated and probably unable to perform his duties for the last year of his term. His wife effectively ran the presidency for a while. But they could not invoke the 25th because it didn't exist.
If Trump was felt to be sufficiently mentally disordered then I think that is covered by the 25th even if he doesn't do anything impeachable, at least in theory.
Trump would have to be so mentally disordered that he was
unable to perform the duties of his office. I'm not saying that is impossible, but being narcissistic and stupid clearly doesn't qualify according to the people that can invoke the 25th.