Welcome pt,
I would try to get at least 4 inches of compost added to the clay but more if you can. I don't know about Charlotte, but most city compost and mulch programs are great. You can find a fair amount of plastic trash mixed in but it's hard to beat the price.
In Raleigh, I've been having good luck leaving Dahlias in the ground and mulching. There's always the chance of loosing them if we have a real bad winter so if they're special to you you might want to dig and store them.

Hi pt and Welcome!
I've come to accept the red clay as a good start to a great garden. Yes, it does take years to blend it with enough additives to give it tilth but it does come about if you avoid two things.
Don't add sand and don't try to till it when it's moist.
This is the perfect time of year to collect the deadfall leaves into a pile and mow the heck out of them to chop them to bits. Add that to your soil (not just layer..actually till them in). I also used Black Kow composted manure (bagged) and many,many bags of good old Lowe's soil conditioner.
Find some quantity of cheap (as in Big Lots) potting soil and dump that on too.
I've never used city compost because I'm afraid it has gobs of weedseeds and seeds of wild bermuda grass. All the coffee grounds you can find also go on the ground to rot and be turned under the next year. Banana peels..on the ground also to dry out and rot.

The red clay acts as a binder to all these additives and if you're careful not to till or turn the soil when it's wet you will within three years have a good,moisture holding but well draining soil to plant in. Add some agricultural powdered lime and a touch of Ironite to the top of the tilled soil and let it set over the winter,uncovered.
I'm a strong proponent of bare ground planting..i.e. no mulch. With all these new additives you want a good oxygen exchange and you want the soil organisms to break down the rotting leaves and other organics.

I would dig and store the dahlias even if they're in a raised bed with great drainage.
The tubers attract voles.
P.S. if you've arrived from somewhere Up Nawth know that down here we don't use peat as a soil additive. It, plus sand plus red clay makes for nice bricks, not nice crumbly soil.