Perfectly OK to use bare wire for ground. Thick gauge best. But I don’t see any reason why it’s better than insulated wire of the same gauge and composition. Nor should the dielectric make any difference. If I have some heavy gauge 3-wire Romex cable lying around, I like to strip out the bare ground wire and use it as ground wire in components. You can get 12- or 14-gauge Romex at any good hardware store.
