“They can be impeached even AFTER they leave office.”
Not legally. The “impeachment clause” in the Constitution reads as follows:
Article II Executive Branch
Section 4 Impeachment
The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors.
The express purpose of impeachment is to remove an officer from office, which obviously can’t be done if the impeached officer is no longer in office.