I did it!
Score: 502,000
I was not as keen on "farming" humans this time around, so had a shittier pace compared to my last game, but in the end I succeeded in my goal.
The best points definitely come from getting humans from martians for 1k or 1.5k points. Later levels give 900 each per remaining human at the end of the level so that's decent, but those extra 100 and 500 points add up over the 35 levels you have available to clear. Could easily have gotten even more points, probably up to 20k or so more points in the end, had I focussed harder on doing so.