Erg mode with power match will always be a bit slower to react than when power is provided directly from the trainer. As others have said, don’t chase power, just maintain cadence. That should dampen the power excursions to an imperceptible level. I don’t find it so different from doing workouts in resistance mode: you also end up all over the place if you try and chase the power target. You set a cadence, see what the power does, and adjust the cadence accordingly.
I also use the same method as @mcneese.chad to take standing breaks: shift to the large chainring while standing up, drop to the small chainring to resume.