I think what you are looking for is 25 cent chickens.
Buy local. go to your local livestock auction that handles poultry, craigslist, etc.
whatever you get, they need at least some protection.
I live in SW FL. I visited a guy who had a real jungle. No pun intended. You felt like you were walking in a tropical jungle. Palm tree, bananas, exotic blooming trees. He had shrubs and bushes and ornamental plantings and a huge koi pond. He had barns, goats and who knows what else. Just imagine
5 acres of tropical jungle. What else he had were chickens in a huge pen with numerous coops for his egg-laying Marans. Very impressive!!
To get to your desire to have a sustainable bantam, this was an ideal place for them. He had about 100 or more feral bantams that lived on his land. No care from him other than what they gleaned from the other animals. They were wild. All he could tell me is he started with a few odd bantams who slept in the trees and took care of themselves. Many die from various causes, like disease, accident or predators, but enough survive to make more. He says the turnover is heavy because he doesn't see the same chickens for very long. Of course, he gets no eggs because they hide the nests.
I guess you cannot have everything. But, the chickens fly, sleep in the trees, don't peck each other and they brood enough eggs to sustain their population.
Hope that gives you an idea. Make sure you have a jungle.