And I strive to maintain heritage breeds so I only have heritage breeds and want to avoid having cross breeds as much as I can.
If you are determined to have only heritage breeds (nothing wrong with personal preference like that) determine what heritage means to you. For some people it is a bird that meets show chicken standards, some may want it to meet show chicken standards plus the behaviors and production qualities the breed was meant to have. I'm probably the only person on this forum that when they think of heritage chickens that they think of the chickens that our great-grandparents kept, flocks that followed the general breed standards but were certainly not show chickens. Instead they were used to feed the family efficiently. That means they would pretty much feed themselves (weather permitting) and provide eggs and meat but would not win any chicken shows. Production qualities were more important than meeting a breed SOP. Personally I use mixed breeds, keeping the ones that best suit my goals for breeding and eat the rest. You can do the same with one breed, my personal preference is to have mixed colors/patterns. If you are going to do that I'd stick with one breed, it can get complicated to raise different breeds and keep them from cross-breeding. Keep it simple, why make your life harder than it needs to be.
Here's my thought process: I'm thinking I can purchase some eggs from a hatchery or something or other of a dual purpose breed like australorps or plymouth rocks,
This is doable. If you are getting them from a hatchery I'd get day-old baby chicks instead of hatching eggs. That way you can better control how many chicks you actually get. You can get straight run (no telling how many will be males or females). I did that the first time I ordered but after that I went with sexed chicks. You really don't know how many of either sex you will get. I got seven pullets from seven straight run Buff Orpington I ordered. I got other breeds that did have cockerels so I still got a rooster. That convinced me that if I want males or females I better order them sexed.
I suggest you order at least 50% more pullets than you think you need, some probably won't meet your standards but still taste great or you may lose some. Get several cockerels even though you plan to keep only one. There can be a huge difference in the quality of cockerels from the same flock, especially from hatcheries. I once raised 18 Buff Rock cockerels from a hatchery and only had three that met my standards as far as being potential flock masters and breeding roosters. I ate all but one. If I had only ordered one which would I have gotten, one of the three or one of the fifteen?
You can get chicks from a feed store, they originally came from a hatchery. Many people do. Depending on the experience and quality of that specific stores personnel you have a greater chance of not getting what you want breed or sex due to mix ups. Some store people are pretty good, some not so much. This way you avoid the hassle of shipping.
keep a rooster and 4-5 hens for breeding year round (or as long as they will), give them safe housing away from the cats and coyotes, incubate all of their eggs and raise the chicks keeping pullets to replenish my egg layers (dispatching hens at about 3-4 years old to make dog food) and keeping the cockerels to raise for meat throughout the year. I would give extra eggs and meat to my parents and in-laws so that there is no waste.
Obviously if you are going to give extra eggs away you are not planning on hatching all of them. I had to read that twice. I think you plan on having an incubator since you mentioned possibly starting with hatching eggs. To try to keep my post shorter I'll go through how I manage this. I know this is way too long anyway, partly because there are so may different ways you can do this.
I typically hatch a bunch of chicks in January/February in my incubator. I'm limited on freezer space on how much I can store at a time, partially because I freeze a lot of stuff from the garden. If you pressure can the meat instead of freezing it all you can increase storage. I typically hatch about 20 chicks so I will have meat available when my frozen meat runs out. After that hatch I might do one more incubator hatch if a hen doesn't go broody early enough but I have several that typically go broody in late spring and summer so I can mostly rely on broody hens after that. If you hatch eggs from hens that go broody and keep the daughters as replacements they tend to go broody too.
My typical laying/breeding flock is 6 to 8 hens and one rooster. I've determined that I need to hatch about 40 to 45 chicks every year to keep us in chicken meat. There are only two of us so even a pullet will make two meals, though the second meal is soup. I keep back 4 pullets to add to my flock each year, keep 3 or 4 of the previous year's hatch through the winter, and eat the older hens from the year before that when they start to molt and stop laying. Since I also play around with genetics I usually replace my rooster annually with one of the cockerels.
I butcher all my excess cockerels and pullets all through the year. I typically butcher the cockerels when they hit about 23 weeks of age. That's my sweet spot the way I feed them and the way I cook them. Others have different ages for butchering, depending on how they cook them and other factors. I typically butcher excess pullets around 8 months of age after I've evaluated them for laying. Some people only eat the cockerels and sell started pullets, they may be able to pay for a lot of feed that way.
One of the big issues of sustainability is how you feed them. If you feed them everything they eat you either need to grow a lot or spend a lot of money buying feed. If you have good quality forage they can find a lot of their own food but they can be vulnerable to predators. It can be a very tough balancing act.
There are so many different ways to do any of this, trial and error can be a great teacher. I'll quit typing for now and see what other responses you get or questions you have. Small farmers have done this for centuries but most of us are not small farmers. Your specific conditions will have a lot to do with what works best for you.