Hi Morg,
The set up for the whole scene is crucial. Hence why I always need a scene file. Screenshots are too simple, thanks for your efforts, but the truth is in the project-file. I have sent you an upload link.
I considering guessing as un-professional. Over the past 15 years, it has proven over and over again to be a time-waster. This includes just answering from memory, as things will change.
However, some standards need to be known anyway:
You certainly need to watch this series, but since you have a specific question, perhaps that is the part required right now.
https://www.cineversity.com/vidplaylist/r15_global_illuminations/global_illumination_part_08
As well as
https://www.cineversity.com/vidplaylist/r15_global_illuminations/global_illumination_part_02
Maybe take a look into the manual here:
https://help.maxon.net/us/#VPGISETUP-GI_GROUP_IRRADIANCE
In short and simplified, real-world light has an infinite number of bounces practically. Nothing that any computer could do (considering what is typically available even in large studios.) Any (!) Global Illumination algorithm works with a minimal set of information that real light would produce. The spots that you see are based on a very few of those. Which is excellent for the setup time. Then we need gradually more of those to get a clean result. The result is an interpolation among all these spots.
The spots are a result of random explorations of the scene. There are ways to support that, especially how the application knows where light sources are. Hence it is not just about a few settings.
Please watch the whole series from above and follow with a very small set up to have a practical experience each time. Just watching it will have a very short time effect only.
The more you know what is going on, the more you will make decisions based on that, and that will get you to the sweet-spot of a good, but also efficient render.
Each scene will have a different set of requirements. Hence why tips, in general, will not work for each and every scene.
All the best