How do you set the slider thumb to a value/step with a trigger?

It seems to me that you want the layers to show when the slider moves, and the thumb to jump when the learner releases the thumb. That's a difficult ask, because updating the variable when the slider moves, and updating it when the slider is released are mutually exclusive actions.

Nevertheless, take a look at the attached sample.

 The slider jumps slide has a couple of methods that might work. Moving to layer 1 shows one option. If you stop moving the thumb, and don't move it again before you release the mouse, the thumb jumps to the corresponding triangle.  Moving to layers 2 - 4  (and from 2 back to 1) shows a different method that I think works (marginally) better. Personally, I don't like either one, because when the thumb jumps, the learner loses control of it.

I much prefer redesigning the slider, using visual clues to show its location. See the Zones slide.

At any rate, if you find anything useful, you are welcome to it.