Simple, don't wait 5 months.
Waiting that long reduces the means you are able to cook a bird. It's gaining flavor as it ages but it's also getting tougher. You can only roast 6 month old birds, you can stew them too. If you want high heat cooking like on a grill or broiling then butcher no later than 14 weeks. If you like southern fried you can wait as long as 18 weeks of age.
The other thing to keep in mind is body structure of most dual purpose birds is not what you're used to. Leg meat is dark. Older birds leg meat flavor is closer to duck than chicken flavor. The breast meat is thin. You will not have that ultra thick breast like a CornishX or slower growing hybrid meat bird. In a nutshell dark to white meat ratio is far different than you are used to.
If breast meat is key to your culinary taste and you still want a sustainable flock of meat you should look into obtaining Cornish from a breeder. Dorking has a little more breast than typical dual purpose birds too but nothing compared to Cornish and hence foundation of all hybrid meat birds.
If you have settled on a Dual purpose, Cornish are not dual purpose as they don't lay many eggs (read- you'll get enough to hatch out another generation each year but little more), there are several at the top of the list for growth and fleshing quality. The afore mentioned Dorking, New Hampshire, Buckeye, Red Sussex and maybe Australorp.
Any dual purpose can work really but those above would be considered better. Regardless of what you use you certainly want to obtain breeder stock. Standard bred birds will have the carcass qualities meant for dual purpose. Hatchery stock do not, plain and simple, quality dual purpose with vent for meat can only begot from breeder stock.
I'll go even further as this thread is likely to be inundated with breeds that really are not better. Many may attempt to suade you on Orpington. It's an OK dual purpose, Australorp were bred from the original black Orpington with eye to utility and why I suggested them. They are not to the standard they once were though. Also many will attempt to say French breeds are the best. This is untrue also. Let's take Bresse for example. It's a culinary delight in France due to fattening sheds. This is where you keep the birds from moving and force feed them concoctions of malasses and corn or the like. It's the fattening that makes them a delight to eat not the breed itself. It's unlawful to do this in America, not to poultry and not to veal anymore.
Since this is already turning into a novello perhaps I should add that a slow growth hybrid may be more suitable to your desires. Instead of rapid growth CornishX you could look into the Ranger type hybrids. Designed to be culled at 12 weeks opposed to the 8 weeks CornishX do. That way you have a more active bird that grows slower so health issues are not of concern and still double breasted- breast meat twice as thick as dual purpose birds. CornishX is more like three times thicker.
Good Luck.
