I’m having the same case. I found a workaround that could work for you.
Generally it seems that cluster N+1 will not break until cluster N is broken, no matter the force. I set damageThreshold[0] = 500 000, damageThreshold[1] = 100 and hit it with a force of 200k. Nothing breaks.
So as a workaround I modified the clusters. Moved cluster level 1 to level 0 and level 0 to level 1. Then changed damage thresholds too ([0] = 100, [1] = 500k). As a result the smaller parts fell of first. This will not work in all cases, depending on what effect you want, it might not be doable.
To modify the clusters, select them and on the Fracture menu use ‘Level Up’ in the Cluster section. The option to reparent (cluster down) didn’t work for me. Instead I drag and dropped lower level onto higher level in the Fracture Hierarchy window.
Also note that it sems that the entire cluster gets broken if any part of it receives damage (Does Chaos have any concept of damage accumulation? or LoS/self-field blocking?) so it gets really hard to setup something different than ‘top down’ destruction.