What times will you be traveling from the campground to the reunion? That will help determine the best campground to choose. I live in the area and know the traffic patterns.
But without knowing that information, James Island County Park (JICP) would be the best. Usually you are either going against rush hour traffic or across the main grain so not bad. Oak Plantation would be similiar traffic pattern, but not as nice as JICP.
KOA N. Charleston would be the shortest and easiest commute, but it isn't usually the place I recommend unless convenience is the main goal.
KOA Mt Pleasant while nice, would have the worst traffic since a lot of days I526 is very slow going for a couple hours. But, your schedule may make this the best choice.
Shame you are not retired, as the FamCamp off of Redbank would be idea. If any of your buds are retired, you could check to see if they would sponsor your stay. You just may find out one is staying at the FamCamp and can sponsor you. There are usually a number of sites available.
When you figure out where you are staying, let me know and I will send you clear directions on the best route. Most folks follow the GPS but there are a number of main roads you can take to save some headaches. Don't worry, I won't be sending you through small neighborhoods. But after 50 years in Charleston, I know my way around.
2015 Ram 3500 4x4 Crew Cab SRW 6.4 Hemi LB 3.73 (12.4 hand calc avg mpg after 92,000 miles with camper)
2004 Lance 815 (prev: 2004 FW 35'; 1994 TT 30'; Tents)