no, you are guaranteed to become Legendary somewhere between 775 and 1030 activity, not early,
more post than the required minimums, does not an advantage to get the higher rank.
Correct, the point at which each poster becomes a Legendary member is as you said when activity is between 775 & 1030 but
it's predetermined when you create your profile so in that case, again as you said the amount of posts a person makes has nothing to do with how quickly they reach Legendary status.
I also guess that the number of posts has nothing to do with it, when you become Legendary.