How can I open my Downloads folder in one click?
If you have the Downloads stack in the Dock, you can ⌘-click it to open a new Finder window to the Downloads folder.
If you want to create an AppleScript application to put in the Dock that when single clicked opens a new Finder window to the Downloads folder bringing it to the top without bringing other Finder windows to the top also, then the follow line of AppleScript code is all that's needed to be saved as an AppleScript application and placed in the Dock:
do shell script &quot;open $HOME/Downloads&quot;

If you want it to always be in the same location on the screen and set to a particular view and size, you can use information in my answer in question you linked in your OP.
Example:
do shell script &quot;open $HOME/Downloads&quot;
tell application &quot;Finder&quot; to ¬
    tell window &quot;Downloads&quot; to ¬
        set {current view, toolbar visible, bounds} to ¬
            {list view, true, {1475, 871, 2499, 1360}}


Notes:

If the Downloads folder is already open it will just bring it to the front without bring  other Finder windows to the front also.
If you close the Downloads folder it will bring other Finder windows to the front and this is the default behavior and has nothing to do with having opened the Downloads folder with this script.