I know that you can COPY an existing board. So I've created a Template board.
Just wondering if there is any way to change the default "new" board to avoid the above.
Right now, that feature isn’t available within Trello. You can copy a board, as mentioned however at this point in time there is no way of making that a default setting for new boards.
As a native app, Trello for Desktop allows for a dedicated menubar with custom keyboard shortcuts, and the ability to define a Default Board that can open whenever you open Trello for Desktop.
In the app, in the menu for "Boards" to "Set Current Board as Default" press Command-Shift-D or Ctrl-Shift-D.