There’s not a huge benefit from doing the whole machine model thing that I’ve been able to see. There’s a short thread about trying it out here
Just to get Fusion 360 CAM working and driving your Shapeoko, when you do the post select the Post processor “Carbide 3D (Grbl)” and in your machining setups just operation type “Milling”
Here’s another relevant thread. I completely agree with @LiamN…there’s not much of a practical reason, that I can see, to bother with the machine definitions. (Though I’d love to hear otherwise)
Are you just trying to generate gcode?