Yet again, you guys were a great alliance, both 604 and 668. It is too bad about your arm. At least you only need to replace the tubing. And now you can take off the steel and add a beefier arm. Good luck.
Also note exactly where it failed - right where the hole on the top & bottom starts. As you get closer to the fixed point (the end held in position) the force on the metal gets higher very quickly given a small force at the free end.
The hole is a “stress concentrator” - the ‘lines’ of stress (or force) cannot ‘flow’ through the hole, they must go through a solid, so all the stresses focused at the end of the hole, and that being a weaker point, it failed.
As stated, it can be calculated with some reasonable assumptions. This is called “engineering”
Of course, even with double the arm, there can always be a hit that’ll bend it. I, too, hope that your repair efforst go smoothly and you whomp them all in competition…