Teach was 38 pre ts,and they look similar.
There's a 9 year gap between this chap and God Valley. that would put him dying in his 40s, which seems fair, as Roger died in his 50s, which is considered prime in OP.
Xebec was most likely a famous soldier, then at reverie forced to become a criminal after killing most likely an evil admiral, and maybe he saw Imu. Then he recruited his crew/studied the void century before dying at God Valley.
How about this
Blue - Yellow - Red Admirals
Green - Purple - (waiting for Orange) admirals
Black - Grey - White admirals
Black (like in Blackbeard) = Xebec
Grey = murdered admiral
White = Sengoku
Black is the color of evil, death and so on.
Grey might have been playing both sides.
White was noble and just, later rose to FA.
As for his motives, who knows. But likely something linked to Astronomy or history (Teach favourite subjects)