That would be a ridiculous workout to do outside. I wouldn’t worry about the tiny power fluctuations or power in general. Just stop on the pedals and settle in to roughly that area of power. Also, just do the warm-up block of 30/30s and then the blocks of 40/20s.
Personally, I’d rather find a VO2max workout that gives you longer intervals with the same time in zone, like 4 x 5 minutes. That would certainly be easier to execute.