I don't want to use any of the usual things like JSON, XML etc because this has got to be extremely light weight as it's going to be embedded

That's fine. I still encourage you to structure your lightweight script file into (unit-testable) subroutines at the top of the file, with each subroutine having well-defined inputs and outputs and not relying on global data ... followed by a short mainline at the end.

A simple example of this approach can be found in this node.

👁️🍾👍🦟