I think the difficulty levels proportions are wrong.
Easy: 10 secs = 70,000 x 3 = 210,000 = difference compared to medium = +30,000
Medium: 30secs = 180,000 x 2 = 360,000 = difference compared to hard = +40,000
Hard: 60secs 320,000
See what I mean? If you look at it that way; easy is harder than medium and medium harder than hard, imo.
The under 9000 thing is a nice idea, but it's also possible in easy mode.
So that achievement is too easy for hard.