Q.Lilacs
In the south, how much afternoon sun is enough or too much?

Lilac do not preform well in the Southern climate.
It is the
Look for varieties such as Lavender Ladies, Blue Skies, and Angel White, which are low chill varieties.
It is the duration of the heat and length of the winter that is the biggest issue growing Lilac in the South.
An alternative is Lilac Chaste Tree.
https://www.gardeningknowhow.com/ornamental/trees/chaste-tree/chaste-tree-cultivation.htm