I think you'd quickly kill that little router trying to engrave granite deeply, as is usual for gravestones. It's not just a matter of spindle power; you need to use water to keep the tool and stone from overheating and to control the dust, but that makes an abrasive slurry that will get into the moving parts of your machine, if they're not well-protected, and destroy them.
A better approach might be to use a drag-knife to cut rubber masking for the stone, which won't hurt your machine or require a bigger spindle, and then use abrasive blasting to actually remove the stone to the desired depth. This would be a lot quicker as well as easier on your machine.