Ubuntu – Too many Show Desktop extension icons under GNOME Panel

I found a way to do that:

1. Open a terminal and install wmctrl:

   sudo apt-get install wmctrl
   

2. Create a shell script named show-desktop.sh (I put it in my home folder)

   gedit ~/show-desktop.sh
   

   place this code in there:

   #!/bin/bash
   status="$(wmctrl -m | grep "showing the desktop" | sed -r 's/(.*)(ON|OFF)/\2/g')"
   
   if [ $status == "ON" ]; then
       wmctrl -k off
   else
       wmctrl -k on
   fi
   

   and make it executable:

   chmod +x ~/show-desktop.sh
   

3. Create a file show-desktop.desktop in ~/.local/share/applications/ folder:

   gedit ~/.local/share/applications/show-desktop.desktop
   

   Add this text and save (don't forget change the value on <your user>):

   [Desktop Entry]
   Type=Application
   Name=Show Desktop
   Icon=user-desktop
   Exec=/home/<your user>/show-desktop.sh
   

4. Open the dash, search for show desktop and add it to the favorites.

To an extent, you have to adapt to the working environment you choose to work with.

1) a grid of workspaces (3 columns, 2 rows to be precise); You already found the Workspace Grid shell extension

2) the application windows remaining in the workspace I move them to; Using Gnome Tweaks, which is not installed by default in Ubuntu, you can set workspaces to "Static". With dynamic workspaces, workspaces automatically are removed when they are empty, so indeed an application you placed on workspace 3 may end up on workspace 2 or 1 later in the session.

3)miniatures of the workspaces content in the switch modal; Gnome-shell provides miniatures of the workspaces in the overview when you press Super. By default, however, you have to hover the right side in the overview to make the miniatures visible. You can make sure the workspaces are always expanded with the extension "Always zoom workspaces"

4) workspace switcher showing the overview of all workspaces. I do not follow here: in the overview, all workspaces are shown. You may also install a "Workspace switcher" extension that provides a clickable icon on the panel that allows you to switch workspaces from a pull down menu.