Solved! I just modified the central circle to still have the other ones to compare with. Check the difference.
For the central one, instead of using a normal circle, I use the suggested method of combining two arcs.
// .h
ofPath circle;
//setup
circle.setCircleResolution(50);
circle.arc( 0, 0, radiusCircle, radiusCircle, 0, 360);
circle.arc( 0, 0, radiusCircle - lineWidth, radiusCircle - lineWidth, 0, 360);
//draw
circle.draw();
Thanks for your help, cheers!