does it matter that the only 2 points i want to nail down is opposing threshold 20 feet apart . would that not allow the floor enough room to equalize expansion ?

Opposite eachother is the worst case scenario. The floor has no where to go, it's "locked in".

my concern is that the 8 ft strip of threshold will butt to tile at same height w/ caulk line separator . the opposite end is the front door and with south florida rains i would feel safer with the subfloor attached , either glue or tapcons . the distance between these is 20+ feet . is the distance cumulative for expansion , or does the distance actually offer ease of expansion ( averaging out ) . are you saying that if i attach those 2 points expansion will cause buckling across the 22 ft ?

so when you say "thresholds are anchored" are you referring to the floating sub-floor ?