score:3
The bus that leaves Saint-Georges drives past this point: 4.599891, -52.404094 This bus leaves a 'few' times per day.
That's as close as you're going to get with public transport, which is still some, what, 7-10k out.
Not knowing how familiar you are with French Guyana; If you would get out at that spot, it's likely that passing cars will offer you a ride. It's fairly close to Cayenne, but few cars ply the road between Cayenne and Saint-Georges, and even fewer will go to Cacao.
Renting a car in Cayenne is a sensible thing to do. Cost can be about 40USD per day, if you drop the car off in the place you picked it up from.