He’s our villain, or some version of his ancestral line is, throughout the trilogy. It’s only Old Biff in the second movie, the time traveler, who has any brains. Every other version of him is mean or subservient. Does he even have an Enneagram, or is he just a generic antagonist?
Actually, I think that’s our answer. Old Biff has a motivation: I want to change my destiny. He’s the only one who isn’t a cartoon. He has a fatherly patience with his younger self. Obviously he’s pretty smart. In just a few seconds he understands all the ramifications of time travel, as well as the motivations of Marty and Doc. That’s why he warns himself to look out for them in the future. A little too convenient? Do we believe that bully Biff can really become this wiser Biff? But then he leaves the almanac bag and his broken cane handle behind in the car, so how clever is he?
Nope, he’s still just a plot device. No Enneagram.