Rear wheel spokes breaking...

To date 4 spokes broke on the drum side in separate episodes. All spokes snapped at the hook end. The last one broke even before I'd refitted the tyre!!

I studied the problem very closely. They must be suffering stress, beyond their endurance limit.

I noted the flanges are a different thickness on each side but yet they take the very same spokes.
My solution is to increase the thin flange (drum side) to the same thickness as the other flange. I effectively achieved this by sliding a washer on each spoke before inserting it into the spoke hole.

Pictured: Spokes assembled with washers.

The washers are 1.1mm thick.

The drum offset to rim edge is now reduced to 18mm. This gives a slight improvement to stability over the previous 20mm, so riding "no hands" is even better.

OK, so it's only been 150km so far but the spokes lines look better as they have a much more of a "straight pull" from the spoke holes.

Hoping this concludes the saga on what's been a very frustrating time. We'll see...

Update: nearly 400km now and all is well.

Update 28 August 2020: No more broken spokes.