Why the Deutschland class could neither fight nor flee

Would a faster 6- or 8-inch commerce raider be better than the Deutschland class?

Yes; Deutschland had useful range but not enough speed to escape larger ships once caught.

A commerce raider must either defeat its pursuers or reliably outrun them. The Deutschland class offered excellent range but lacked enough speed to execute the second option against larger warships. Its heavy guns did not compensate once a superior force caught it. A faster 6- or 8-inch cruiser would suit the raiding mission better because escape is more valuable than nominal hitting power on a ship that cannot win the resulting battle.