There's a reason most of the inland south was lightly populated from the very beginnings of this nation. Year-round growing conditions exist in subtropical to tropical areas. Disease. To help paint the picture, as a college student I worked summers at a gorgeous old mountain lodge and resort that had it's beginnings as a summer retreat for the well-to-do of Charleston and Savannah. It belonged to a former Confederate of note, Wade Hampton. They fled the "foul miasmas," yellow fever, mosquito borne diseases. The old money set from those locales still summer there, despite the eradication of the disease that set their ancestors fleeing. Tradition and all that, southerners are big on it.
Further inland, there is no year-round growing season. Winter is much shorter and less severe, true. The soil tends toward red clay. You can grow a great garden in it if you know what you're doing, but it takes work and soil enrichment. Without that, red clay makes excellent brick and that is exactly what it seems like after baking in the hot summer sun. I've actually had a pickaxe bounce off of it before, when I was trying to install a dry laid fieldstone wall one July. I ended up waiting until October, lol.
A mild temperate climate with enough rain or surface water to avoid drought conditions would be best, and soil type matters. Silt loam, brown to black. Hard to find that in mild temperate zones. Riverbottoms in the far southern Appalachians, especially in the "thermal belt" that rarely gets snow on the ground would work. The cities there have gone way left for the most part, see Asheville, NC. The more remote areas are either foo-foo resort or very clannish mountain people who would view you with suspicion.
Excellent post, Regulator. You bring up the matter of food growing problems in that area.
Soil, as you note, can be amended.
Now trying growing a radish in the northern tier of states in February or March. After you dig through the snow depth, pick off the bottom ice layer and get your drill out to make a seed hole into the soil freezeline, you might, if lucky, see a radish in mid-June.
In the meantime, people in the southeast will be eating blackeye peas and collards - not great but it is food - while their northern cousins are munching snowcones.
Survival in the southeast is much more probable than in the mid-west or northeast. And those areas are also infested with vast swamps. Both Milwaukee and Chicago were built on swamps.