I think you will find the reason it breaks off is because it is tinned a fair way up the wire and so offers no flexibility therefore it acts like a solid strand of a single wire and has no give.

Vibration and movement cause flexing of the stiff, tinned wire and so it snaps.

It really doesn't have a lot to do with the gauge of the wire used IMO more HOW it has been connected.

I feel that most harnesses are inadequately supported with cable clamps etc and wires are just left "hanging in the breeze" this is the main cause of wiring problems as I see it.

Perhaps use 20 or so small cable ties and prevent the wires from flapping about would do more than replacing them?