We're now feeding grass pellets, stocked by most pet and horse places as well as agri merchants. The Dengie one is meadow grass, nothing added, non-GM, 16% protein.
Or you could mix that with the goat mix and so dilute the copper
Copper-sensitive breeds include North Ronaldsay (highly sensitive, even a little can kill a copper-naive sheep) and Texels. I've never been sure whether it's just that Texels are really good at managing on very low copper, or are actually likely to die from over-exposure. But even if it's the latter, I believe it's a cumulative effect, so they're unlikely to have their livers explode if they're fed goat mix for a few days.