Ok, I was able to figure this out. Or at least the next step...
I was confused because ScrollTrigger's .progress() function is read-only. I couldn't figure out how to update it. Then I stumbled upon the fact that I didn't need to update the ScrollTigger; I just needed to update the tween. I'm still new at this stuff. but I think it makes more sense now.
I updated...
function onDraggableDragHandler() {
const draggablePercent = this.x/this.minX;
const markerPx = marker.maxX * draggablePercent;
gsap.to(markerElem, { x: markerPx });
}
To...
function onDraggableDragHandler() {
const draggablePercent = this.x/this.minX;
const markerPx = marker.maxX * draggablePercent;
gsap.to(markerElem, { x: markerPx });
scrollTween.progress(draggablePercent);
}
It seems to work pretty well as I intended. Though the .marker needs some smoothing.
Agency Habitat's post in Update timelines when dragging was marked as the answer