We use 26 when going to and from Charleston. Your only problem on 26 will be boredom. That is a long, flat road to Columbia. We usually go I-20 to Atlanta from there. 20 would put you in Atlanta around mid-town and a lot of traffic to get up to Alpharetta.
Depending on the time of day I-85 can be challenging (daylight to 10 AM or so) coming south into metro Atlanta.
Pleasant Hill Rd off 85 will take you into Alpharetta.
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