Your Garmin head unit should be set for at least 10 sec (time constant), if not 30. You can probably go to some website like Training Peaks, Xert or other, and change the time constant then import it again into GC. Sounds complicated though. I don't understand why you are unable to change the time constant on your head unit. But then again, maybe the exported file is always unfiltered - in which case it does seem like Garmin Connect should provide a smoothing algorithm that you have some control over.