We do have a 25th Amendment, which contains a section providing a pathway for dealing with a President who is incapable of carrying out the duties of office.
It was most likely put there for the event of a massive stroke or other major medical incident that would put the President out of commission, but could be applied if the President was demented, insane, or otherwise mentally unable to perform.
Unfortunately, the latter is more difficult to deal with, as it requires support for removal on this basis from the Vice President, a majority of the President’s cabinet, and two-thirds of both the House and the Senate. And this makes application of the 25th Amendment highly unlikely, as most of the above are Republicans who don’t want to admit they have a loser.
Impeachment will be easier, as that only requires a majority in the House to impeach, and two-thirds of the Senate to convict and remove from office. And while there’s already a pile of possible offenses that are impeachable, I suspect more ammo will be coming when current investigations are concluded.