I'm going by bus from Saint-Georges to Cayenne, passing through Roura. Is there any kind of public transport or scheduled service from any of those places to Cacao? Rome2Rio and other sites recommend going by car, but I was hoping to find something cheaper.
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.