Timothy
A couple of observations from flying VFR in the US and watching electrical activity on a lighting detector (not WX500) and Wx radar from a safe distance. I was flying with a turboprop FO, who was also a flight instructor and he was teaching me some aspects of flying VFR around build ups.
(1) CuCb with appx 12-15,000' tops, with intense electrical activity and lots of red painting on the wx radar
(2) Cb tops above FL350, little electrical activity, although the Wx radar was painting plenty of yellow areas and occasional red
I don't know if this helps you at all, but the instructor said that these examples showed why US IFR pilots value lightning detectors so highly, because "cloud size does not always equal meanness." They are also suspicious of attenuation on Wx radar.
Maybe a few more experiments are required to build your confidence levels in the WX500?
I can certainly understand why you wish to build your experience of the new piece of kit, before relying on it.