My two cents.
Don’t bother with this. Do something more interesting like hiking around interior of Pingdong or Miaoli or Chiayi Or just do sections and skip the crappy bits.
Much of these round island routes are horrible walks or cycles.
Also there was some insane talk about hiking the spine of Taiwan. Maybe on the roads but not over the mountain tops themselves.
Are you actually talking about walking along the side of the freeway?
That’s illegal and you’ll be picked up by the cops pretty quickly I would say and actually you will die because trucks or vehicles will use the hard shoulder from time to time.