I've fixed some issues with the source code relating to the problems from my last tests. To see if there was any improvement I threw the Fusion 360 G-Code on there and this happened:

Complex shape cutting success! On the third pass down into the wood I noticed the Y axis had drifted by maybe half a millimeter though, so I stopped it there. This has happened on some of the other tests too, but the other two axes seem to be perfect. I'm guessing this must be some sort of mechanical issue, so I'll need to try and figure out what is happening there.

The Arduino is also stopping to have a little think in between commands also, which is a problem when the program generates lots of little curves like on this model. There is probably a little optimisation to be done there.

Super happy to be finally cutting out exciting things though, I'm starting to feel very close to having an actual useful machine!