Keep in mind that here all GW trikes average between 11-13 k miles because of chip and seal roads, and the constant twisties.
I am one happy camper. YMMV.
I read an article years ago (60's) in Popular Science or Mechanics that compared tire wear by each state. It all depends what filler they use in the asphalt which depends where they are located and what is more available and cheaper to the pavers. I remember the SE being very hard on tires due to granite in the asphalt they use. Florida, where I'm located used coral rock which is very soft and gave good tire wear. I have over 22,000 miles on my front BT45 and I'll probably get another 10,000. Of course we don't have the twisties like you do unless you count entrance ramps to interstate hwys.