According to the Sunflow documentation, only the PointLight and TriangleMeshLight types are attenuated light sources.
By ambient lighting, it is necessary to understand the minimum lighting so that a room is visible without artificial lights.
Take the example of your closed room with a rendering in quality 4 (default values).
I add 2 light panels of the same thickness as the cornices and the dimensions of these grouped.
Panels halfway up the ceiling (example 1)
Panels up to cornices (example 2)
Enko