If you ever have to hide dark-colored chicken meat, try putting it in a soup with black beans. The beans color everything, so even normal chicken meat turns gray in a dish like that.
They will probably not be "big enough" by 8 weeks, but you might just try one or two anyway to see if that is what makes the difference in the quality of the meat.
As for whether it is mostly bone, that depends a lot on the breed. Some breeds do grow mostly bone first (Brahmas are especially...
Cornish cross are often butchered at 8 weeks old. I think the age difference may be more important than the choice of breed.
So I suggest you try butchering one or more birds at 8 weeks to see if the meat is acceptable then. Just pick a few cockerels out of your next batch of chicks and see how...