I seem to have some small issues with reliability at the moment that I am trying to sort out. I cut a little scale on a piece of scrap to check that the proportions are correct on my axes, which came out perfectly:

But the rest of my testing was a bit hit and miss:

I initially tried to feed it some G-Code generated from Fusion 360 (top right) but it got a bit confused on one of the curves and started wandering off.

I then did a few curve tests feeding instructions one at a time(top left), which seemed to work fine, so I then decided to test the repeatability by cutting the shape on the bottom left all the way through in 1mm increments. I got 7mm in when suddenly it started to go off script and cut diagonally into the material. At that point I shut it down.

I think I'm going to need better methods to debug what is happening in the Arduino when something weird happens. I'm also going to check my parsing code with a wider range of data to see what happened with the Fusion 360 G-Code.