We’re currently using modified point buy for our West Marches campaign. A single score of 8 can be reduced to a 6 to give +1 to another stat.
Next campaign, I’m considering 4d6 kh3 five times and then, for the sixth score, take those 5 low dice and kh3.
Alternatively, 3d6+1d8 kh3, and follow the above for the 6th score.
Doesn't...that just effectively guarantee that your sixth stat is 9 or less?
The odds of getting every die in 4d6 between 4 and 6 inclusive is only 1/16 (=.5^4). That means, for any given set of 5 rolls, you have 1-(15/16)^5 = 0.2758 or about 27.6% chance of getting
any dice in the "5 low dice" set. Almost three quarters of the time, you'll be having a set no
better than {3,3,3,3,3} and almost surely worse, so the absolute best you can do is 9 the vast majority of the time--and that's only if you get relatively lucky and have at least three 3s.
I'd have to crunch numbers a lot harder with the mixed dice, but the overall chances of the lowest die wouldn't change
that radically (after all, the d8 has a 25% chance to simply be superior to anything a d6 could roll to begin with). It is, of course, obviously more friendly, but the impact on the sixth stat will be pretty small as the d8 will very rarely be the lowest die (overall 13% to be THE uncontested lowest die, slightly higher if you allow tied-for-lowest.)