It didn’t occur to at the time, but you can simply turn off Use Inverse Squared Falloff on the Light properties to avoid having to overload the light intensities - this will give let you use much lower light Intensity and ILI settings, for more subtle control over the look and in particular less GI overloading from indirect light.
Sadly I’ve no solution for coloured light rays - direction lights can’t clamp (I don’t think) the Light Function texture, so although you can use Light Functions with them, they’ll be a repeating tiled projection. I might look into this more this evening.