Keithl, I interpret the problem as wishing to cross a line of storms (ITCZ).
There will be situations where the overall weather and storm pattern is such that you should never contemplate a crossing; this requires the best information before departure and en-route.
However, situations are never that ideal, thus there is need for considerable in-flight assessment; use of the best aids, WXR, ask others, etc. This also applies to conditions where in might be justifiable to consider a crossing.
Thus this is a judgement issue, the skill of which should improve with experience in the vicinity of such conditions, knowledge of the hazards (risks), and any other constraining factors, e.g. fuel.
Passengers are irrelevant – my safety (the required management of risk) is invariant to either pax or cargo load.
Perhaps the answer is to never knowingly enter or deliberately plan to enter a Cb when there is another option – an appropriate baseline bias. Always have another option, undo, back, Ctrl Z.
One view of expertise is being able to ask the right question (because the answer is then obvious). In this instance the first question crews should ask is “should we be crossing this line of storms”, and only with a well reasoned yes answer consider how the crossing could be done safely, minimising risk.
My experience north of Nairobi, faced with an ‘enormous’ storm (I had never seen such a large cell either before or since), was to deviate off track by 100nm. After landing I was invited by E African control to explain my deviation, and (to them) the unannounced flight path – the hazards of communication in such conditions, but this action was judged much safer for everyone than contemplating flying any nearer to the cell.