IMO, Letting a denuded slope fill in naturally, is taking an erosion risk. Which can be expensive... reseeding or even regrading if the erosion is severe.

I would get a native seed mix from a reputable nursery, combine it with the wheat straw or annual rye. Your contractor can hydro-seed with the native mix.

And if the slopes are real steep I would cover them with an erosion control blanket, which holds the seed and the soil in place until the vegetation gets established. The cost of the blanket is considerably cheaper than regrading and reseeding.

Here is a link that might be useful: Native sources in Georgia

sun or shade?
If you are looking for bulk seed this link is a good source

Here is a link that might be useful: swallowtail gardens

I think there are a couple of options. One is to apply erosion protection that does not grow, such as an erosion control mat, jute netting, straw, or even hydroseed mulch without any seeds in it (hydroseeding generally involves spraying a mixture of a sticky organic matter plus seeds. the sticky stuff holds the soil and seeds in place until the seeds sprout) If you go with the hydroseed, you'll want to make sure that the tank is clean before they fill it with your mixture or you get some of the seeds they used last time the tank was filled. This would, as you suggest, probably allow a little more erosion than seeds would allow, but shouldn't be too bad if done thoroughly. Then you could either plant or allow wild plants to fill in.

Another option is to use a plant that will not make it through winter. Up here in PA, I could seed oats in late summer, get a nice thick growth of oats, then they would die before setting seed, leaving a network of roots in place to hold the soil. A little research should reveal if oats are winter hardy in your area, and if they aren't then you might try them, probably combined with straw or hydroseed.

Don't fertilize the soil is you plan to grow natives. Fertilizer will benefit weeds much more than natives.

If you plan to seed natives, you could use short-lived plants like Clasping Coneflower, Black-eyed Susan, Monarda punctata, and Standing Cypress (Ipomopsis rubra) all of which will reseed for a few years, but eventually give way to natives from the surrounding area. I definitely wouldn't buy a wildflower mix except from a very reputable source. many so-called wildflower mixes include lots of non-native plants that many native plant enthusiasts would consider weeds.

Finally, I'll say that I think the plan of spraying straw, allowing leaves to fall, and letting nature take its course is a good plan. You'll probably have to remove some weeds in the next couple of years, but as site ages you should see fewer and fewer weeds and more native. If you have spots where the soil washes out repeatedly, you can stabilize those spots with an erosion control blanket, but most of the slope should be fine.