After a lot of trial and error, new projects and in the midst of writing a response with my findings I noticed something in the detailed error message:
In the process of removing anything and all that could cause an issue I removed one of the Node2D nodes and just used the root node (Memory). Yet, the error message refers to "Board" which I thought was an odd reference to the script. But, once again, no other nodes existed in the project, especially with the name Board, since I removed it.
It then dawned on me that I had created a Global variable for the script in the project settings which had that name. Once I removed it, the timer worked! I also realized that my code was running twice as a bonus.
Firstly I didn't realize that the global was actually running the script automatically (yes, despite the Autoload label), I just presumed it made the script accessible throughout the project. So, for the sake of learning, was it running a separate board.gd script and couldn't find the node because globals don't have a connection the the node tree?
Either way, thanks for the help - off to finish this game for my 3 year old !