If I was designing a machine for carving granite, I'd want to protect everything with bellows, since the dust and slurry is abrasive and gets into everything that slides, ruining it quickly.
It seems like that spindle is somewhat light-duty for granite; I'd want something beefier, especially if I was using it to power a saw blade. Or is there a separate saw arbor and milling spindle?
Granite-cutting machines I've seen have all been big and heavy. Is there some special reason you're making yours small and light?
Fusion 360 doesn't support 4th, 5th or 6th axes. Are you planning to migrate to different software when you add the extra axes?