for dynamic custom toolbar, we need access toolbar from quill instance in javascript, this topic show how to access toolbar via quill instance and dynamic add button to toolbar

{ Access Toolbar from Quill Instance }

quill.container.previousSibling.

{ Dynamic Add a button to Toolbar  }

      //create btn
      let button = document.createElement("button");
      //display button text
      button.innerHTML = ">_";
      button.onclick = function() {
        //action code here
      };

      //create btn container
      let buttonContainer = document.createElement("span");
      buttonContainer.setAttribute("class", "ql-formats");
      buttonContainer.appendChild(button);
      
      //add to toolbar
      quill.container.previousSibling.appendChild(buttonContainer);