Quick test with the new Caustics include/exclude: These are all the Caustics element:

Great progress! But some thoughts and issues:
As you can see the caustics are significantly reduced on all objects not in the "include" list (the pool geometry/water etc.), but they're actually still receiving caustics, if you look at the exposure-cranked versions on the right. I think this
might be because they are still appearing on reflective objects? So in other words all those other objects that are excluded might not be DIRECTLY receiving caustics but they are still indirectly reflecting them maybe. I think this is not ideal as really the point is to exclude them totally as a visual control.
Also since this is working through include/exclude list it doesn't work at all through XREFs which is a productivity/management killer. Ideally this would work through GBuffer ID and Layer system.
Rendertimes were pretty much identical with/without exclusions (which is expected I guess, but not ideal).