what you do in statistics when you have such a signal, you use some kind of filter or smoothening. A smoothing could be something like a moving average.
If you own a Garmin watch which supports custom data fields, it could be an option to find someone who will implement such a moving average HR field for you. I know it’s probably a lot of effort, but it might yield the desired outcome. Maybe this field is already implemented in the connect IQ store.