Automation of finding a starting point of measurement in a large dataset
Well ... I would do exactly what you did. The derivative on original signal is very noisy. I would probably take derivative out of moving-averaged smoothed signal, however it brings some delay into your detection. See this answer for more info and python code.
The other approach is to detect the point in time-frequency domain. Simply plot the STFT of your signal and see if that helps the accuracy of detection over derivative.