SortYoutubeWatchLaterList

sortWL.js

// This worked on 2019-10-09 under Chrome in full screen mode. 
// Did not work under mobile, they changed ... things? 

// You put this in Chrome's Snippets (copy/paste) and then run it from there when you are at the watch later list
// (https://www.youtube.com/playlist?list=WL)
// It will move one video per invocation.  I tried to make it do all of them but $$ disappeared. 

// Heavily borrowed from many places

// function for triggering mouse events
var fireMouseEvent = function (type, elem, centerX, centerY) {
    var evt = document.createEvent('MouseEvents');
    evt.initMouseEvent(type, true, true, window, 1, 1, 1, centerX, centerY, false, false, false, false, 0, elem);
    elem.dispatchEvent(evt);
};

function simulateDrag(elemDrag, elemDrop) { 
  // https://ghostinspector.com/blog/simulate-drag-and-drop-javascript-casperjs/
  // calculate positions
  var pos = elemDrag.getBoundingClientRect();
  var center1X = Math.floor((pos.left + pos.right) / 2);
  var center1Y = Math.floor((pos.top + pos.bottom) / 2);
  pos = elemDrop.getBoundingClientRect();
  var center2X = Math.floor((pos.left + pos.right) / 2);
  var center2Y = Math.floor((pos.top + pos.bottom) / 2);
  
  // mouse over dragged element and mousedown
  fireMouseEvent('mousemove', elemDrag, center1X, center1Y);
  fireMouseEvent('mouseenter', elemDrag, center1X, center1Y);
  fireMouseEvent('mouseover', elemDrag, center1X, center1Y);
  fireMouseEvent('mousedown', elemDrag, center1X, center1Y);

  // start dragging process over to drop target
  fireMouseEvent('dragstart', elemDrag, center1X, center1Y);
  fireMouseEvent('drag', elemDrag, center1X, center1Y);
  fireMouseEvent('mousemove', elemDrag, center1X, center1Y);
  fireMouseEvent('drag', elemDrag, center2X, center2Y);
  fireMouseEvent('mousemove', elemDrop, center2X, center2Y);
  
  // trigger dragging process on top of drop target
  fireMouseEvent('mouseenter', elemDrop, center2X, center2Y);
  fireMouseEvent('dragenter', elemDrop, center2X, center2Y);
  fireMouseEvent('mouseover', elemDrop, center2X, center2Y);
  fireMouseEvent('dragover', elemDrop, center2X, center2Y);
  
  // release dragged element on top of drop target
  fireMouseEvent('drop', elemDrop, center2X, center2Y);
  fireMouseEvent('dragend', elemDrag, center2X, center2Y);
  fireMouseEvent('mouseup', elemDrag, center2X, center2Y);
}

{ 
    videos = []; 
    let allAnchors = $$("a#thumbnail.inline-block.ytd-thumbnail");
    for (let i=0; i<allAnchors.length; i++) {
        let thumb=allAnchors[i];
        let href = thumb.href; 
        if (href && href.includes("&list=WL&")) { 
            // console.log(href);
            // would grab other things about the video here
            let timeSpan = thumb.querySelector('.ytd-thumbnail-overlay-time-status-renderer');
            let timeDigits = timeSpan.innerText.trim().split(':').reverse();
            var time = parseInt(timeDigits[0]);
            if (timeDigits[1]) time += (parseInt(timeDigits[1])*60);
            if (timeDigits[2]) time += (parseInt(timeDigits[2])*3600);
            videos.push({anchor: thumb, time: time, originalIndex: i}); 
        } 
    }

    if (videos.length > 1) { 

        for (var i=0; i<videos.length-1; i++) { 
            var smallestLength = 86400;  
            var smallestIndex = -1; 
            for (var j=i+1; j<videos.length; j++) { 
                if (videos[j].time < videos[i].time 
                    && videos[j].time < smallestLength) { 
                    smallestLength = videos[j].time; 
                    smallestIndex = j; 
                }
            }
            if (smallestIndex > -1) { 
                  console.log("drag "+smallestIndex+" to "+i);
                  var elemDrag = videos[smallestIndex].anchor; 
                  var elemDrop = videos[i].anchor; 
                  simulateDrag(elemDrag, elemDrop);
                  break;
            } 
        }
    }
}

not working anymore

You may check my fork here for a working version:
https://gist.github.com/KohGeek/65ad9e0118ee5f5ee484676731bcd092