Very good points, many of which I've not considered yet. This is simply where I am now. We've got plenty of eggs from our Ameraucanas to meet our needs. We're also adding a new light breed this winter that should be prolific layers. Meat quality is going to be just as, if not more important than cost effectiveness. I'm estimating we would process no more than 150 birds per year if we really got serious about it. I know I want a meat producer that can produce enough eggs for replacements since my other flocks will take care of the eggs. I've got to work on the other points you mentioned.
LOL - I think you've gotten yourself more questions yet again.
So it seems that you are planning on keeping your Ams, and also adding another breed that will be a layer breed. Which means that you could look toward a straight meat breed rather than a dual-purpose breed. In which case my first thought goes to a real Cornish, not a modern Cornish cross. But then too, you may have other needs or desires in which case a dual purpose would work better. There are dual purpose birds that are intended to be heavier on meat production like Brahmas, Jersey Giants, and Orpingtons. The trick there though is to find someone that has been breeding them either for meat production, or toward the true SOP of production and appearance, so that you get that meat production out of them that you want. Orpingtons are a dime a dozen but most aren't much more than feathers since most folks don't want to breed chickens for meat.
And of course you mention quality of the meat. That is another subjective issue.
Any bird you raise at home is liable to taste better than what you get at the store - even one of these modern meat hybrid birds that are ready to eat in 12-16 weeks. Bee swears by her White Rocks as having the best flavor. Other people have their favorite flavors. They even have that Ark of Taste where they hold taste tests of poultry and let people comment on the different flavors of the meat. Back around the turn of the 20th century, my breed, Javas, were considered the finest table fowl. My husband and I enjoy our birds while I've got family members griping because they think our birds are tough. Our birds are not tough, not the way I cook them, but most of my family is accustomed to grocery store chicken - where you cook it just a little bit and it turns to mush in your mouth. Our birds have muscle. Even though they are large birds, they run and fly and develop their muscles, so you actually have to chew the meat. So those family members consider that to mean the birds are *tough* and they dont' notice the great flavor. So you're gonna find a lot of different opinions on taste and quality of the meat. And that still will differ from flock to flock depending on the breeding and the husbandry.
Once you narrow things down, in the end it is likely going to be something else that gets you to try a breed. For me and my husband, we were looking for an old fashioned bird - tough to withstand the crazy temperature and weather shifts we have here, didn't need a lot of pampering to survive, good dual purpose farm birds. And the extra plus was getting one that needed conservators to help keep them going, because heritage is important to us. Other people decide they want a particular feather color, or a particular egg color. Some people decide based on whatever bird survives in the environment and husbandry that they give their birds. You have to like the birds. If you don't like them, you aren't likely to keep them. And sometimes what you like about them has absolutely nothing to do with the goals that you wrote down that you want your flock to have.