As far as meat birds, that is a soap box I will get on briefly....buying Cornish cross meat birds not only supports Tyson (they own the genetics, and all those chicks only come from one of two hatcheries world wide), a company that regularly abuses chickens, but also continues the practice of breeding these monstrosities. It has been proven that Cornish cross chicks are in constant pain for the entirety of their short lived, due to their rate of growth.
If you want meat birds, that's yet another argument for getting into a truly dual purpose breed, that is an excellent egg producer as well as table bird. That is breed for its original purpose, rather than just for exhibition. It might take a bit of doing to find a good breeder that is doing the breed justice, but the rewards for a little bit of leg work will be huge.
I agree with what you write. I could go breed by breed down the list of hatchery chicks we've raised to illustrate difference between them and the SOP. But I'll spare you.
I have done one small batch of Cornish Cross (a feed store promotion), and they were truly delicious and huge and very much an "efficient" choice and just thinking about it makes me hungry for more and want to share with the whole world. But still ...

I thought they were cute and they certainly were "easy," but they aren't "right."
What do you think about any of the other special meat bird breeds? Are any of those more ethical? Any of them "sustainable?"
What breeds do you recommend for Dual Purpose?
Out of the hatchery birds we've raise, the ones that show the most promise as Dual Purpose are the Speckled Sussex and the Gold Laced Wyandottes as they are pretty much the only ones worth eating, but that could just be odd hatchery genetics talking. I've kept a rooster of each of those two breeds just in case I decided to work with them for dual purposes.
And for the big question: Are you willing to share any specific information about where to purchase whatever breeds you do recommend?
I have some people who are pressing me to start a growing group for meat birds, but they are really interested in producing the birds as quickly and cheaply as possible so folded up all their "happy chicken" ideals when I showed them the numbers on Cornish Cross vs. other breeds of meat birds. Dual Purpose breeds as meat birds might be a very tough sell.
We are so spoiled regarding the cost of food here in the USA, and I'm finding many people have a fantasy that if they buy food from a small farmer they should pay less than at the supermarket. Especially if the food isn't "organic."
That said, I have had two customers insist on paying more for the eggs I sell than what I was charging. That was nice of them. But I'm still selling my eggs at a loss. I've also had a couple customers ask about "retired" laying hens for meat.
Related to all of the above: I need to find a better source for chicken food. Anyone care to make recommendations about that?