It seems preposterous to assert that the President in NOT an Officer of the United States, when Article II of the Constitution repeatedly mentions the “Office of President”, “removal from Office, etc.
The simplest reason the 14th A is inapplicable is that Trump has never been found guilty of insurrection, despite a Congressional impeachment inquiry about the J6 events. That was the time or never.
The POTUS isn’t an officer, the POTUS is the executive branch. He IS The Executive. “The Office of the President” is the physical walls and doors and windows of an office at the White House. The President however physically is and represents the whole of the Executive Branch. So no, the President isn’t an “officer” in the normal sense.