My fix ended up being two things stacked together. First, I started pulling my eggs out of the walk-in a good 30 minutes before service instead of cracking them straight from the cold shelf, which made a bigger difference than I expected. Second, I switched to a towel ring under the bowl so it wouldn't slide while I was whisking one handed during a rush. The breaking stopped about week 3 after that and I stopped messing with it.
What was the actual fix though? I've been cooking brunch for six years and I'd bet money it wasn't one single thing. Everyone wants the magic answer like it's always the double boiler or always the butter temp, but half the time it's your eggs being too cold or the bowl being wet or you added the butter too fast because you were slammed. I just don't buy that 11 months of troubleshooting lands on one clean cause. Feels more like you slowly got better at paying attention and now you think it was one thing. What did you actually change?