About Farkle Threshold Score
Here's an analysis of the game of Zilch, whose scoring only differs slightly compared to the scoring you've given:


Three pairs scores 1,500 instead of 750.
You may score 500 points if you roll all 6 dice and have no other scoring combinations
Multiples of the same dice score 100*n^(x-2) instead of 100*n*(x-2), where n is the value of the die and x is the number of dice in the combination.  I.e. 4 twos would score 800 instead of 600, 5 twos would score 1600 instead of 800, etc.
Scoring a "Zilch" (a score of zero) in three consecutive rounds costs 500 points.


The first three rule differences will slightly increase the expected value of a roll.  The fourth rule difference can be ignored by only following the Zilch strategy when you have zero Zilches.