Here we go. This rendering took about 3 hours time on my computer.

Default Production mode, with 1000 finalgather rays.
MinRad = 1 and MaxRad = 1

RESULT:
Looks good, although it is still grainy, but much better than the preview rendering.

Looking at the columns, the sampling can be improved by setting min samples to 1 or higher (warning: higher sampling takes much longer time to render if I recall correctly).