I'm in 6B and have planted as late as Christmas week. However, if you plant in the spring as early as the ground can be worked (which means February here) you should be OK for a regular summer harvest. As a general rule, hardnecks do better in the north and softnecks in the south. But either can be grown anywhere.

Ajuga is only suitable for a small scale groundcover. It often develops crown rot and then dies off in sections. In a small area, it refills pretty fast, but I'd never use it for a large area. And a low groundcover, only a couple of inches off the ground, couldn't hide poison ivy--the PI would outgrow it in no time. I never worry about snakes either. They are more scared of you than you are of them. Anyone moving through an area would be making enough noise to scare off the snakes. I do have copperheads, but they are few and far between and in all my 70 years they've never been a problem. You probably don't have any poisonous snakes in your area and the good ones aren't worth worrying about. And, BTW, what is the existing groundcover?

What's at the base of the rock wall? Sidewalk? Tearing out the root structures that are anchoring the hillside might compromise the strength of that wall. Is that wall contiguous with the two properties to either side of your house? If so, how do they have their sloped properties landscaped? Working on a steep slope is a job for the experienced landscape renovation crew. For now, get an identification of the upper shrub hedge and leave it in place until you put together several quotes from recommended landscape professionals and work up a budget. Yes, early fall is the better time for plant installations. If you're brave, take a long handled strong metal tined rake and see what's hiding under the pile of burlap. Might be as simple as an old tree stump. But, a pile of anything on the ground is a haven for critters and the snakes they attract.

Don't do that! If you're like me you'll come home with a car full of plants and still not know what to do. Lol. Suggest googling gardening on slope and see what you find. Take a preliminary list so you're not seduced by all the pretty colours. I'd start with a couple evergreen shrubs, something flowering for spring and maybe a big grass to give you some structure. Then assess what you'd like in terms of colour, size, form for fill in perennials, annuals and ground over. Plan to buy 3 , 5, 7 of things to avoid spotty look of a bunch of individual selections. Cruise your neighbourhood for what is doing well. Hit up gardening friends for divisions to stretch your budget. Spend a lot of time sitting on that patio and day dreaming about what you'd like to see. That last one is very important. Lol Here are some good inspiration pictures.