When I had to implement a UI for reordering a list, I just had a “move” button on each item, and when you press it “move here” buttons would appear between every item (and at the top and bottom). These buttons are positioned absolutely, so there is no reflow from stretching the list. The location where you ‘place’ the item is where you click, not dependent on scroll position. Without even planning for use on mobile it ended up “just working” on mobile because you only need to tap buttons.