Maybe like this, also adding a linecounter so adding or removing newlines makes it add or remove more than if it's only a normal character.

    function textarea_change() {
      // https://stackoverflow.com/questions/7745741/auto-expanding-textarea
      for (const textarea of document.getElementsByTagName("textarea")) {
        textarea.onfocus = ""
        textarea.onblur = ""
        textarea.setAttribute("charcount", textarea.value.length);
        textarea.setAttribute("linecount", textarea.value.split(/\r\n|\r|\n/).length);
        textarea.style.height = Math.max(80, textarea.scrollHeight) + "px";
        textarea.oninput = function() {
          var textlen = textarea.value.length,
              charcount = textarea.getAttribute("charcount"),
              chardiff = charcount - textlen,
              linelen = textarea.value.split(/\r\n|\r|\n/).length,
              linecount = textarea.getAttribute("linecount"),
              linediff = linecount - linelen,
              charsize = 0.3,
              linesize = 16,
              minimum = 80;
          // reset if zero
          if (textlen == 0) {
            textarea.style.height = minimum + "px";
            return;
          }
          if (linelen != linecount) {
            textarea.style.height = Math.max(minimum, parseFloat(textarea.style.height).toFixed(2) - (linesize*linediff) ) + "px";
          }
          if (textlen != charcount) {
            textarea.style.height = Math.max(minimum, parseFloat(textarea.style.height).toFixed(2) - (charsize*chardiff)) + "px";
          }
          textarea.setAttribute("linecount", linelen);
          textarea.setAttribute("charcount", textlen);
        }
        if (autofill_sage) textarea_autofill_sage(textarea)
      }
    }