I wanted to ask about your experience with the heart rate zone settings. I have a Strava Relative Effort data field uploaded to my Garmin watch so I know the running value as I run. In my watch, I have the zone set in the strava data field, just like in garmin. Directly in Strava, if I leave the zones Based on max heart, the values after the activity plus minus match. If I set Custom herat rate zones in Strava, the values are almost half as low after the activity (in my watch, the relative effort shows, for example, 79, and after uploading the activity to the diet, I have a relative effort of only 45, for example).
Did you set the same custom heart rate zones in Garmin Connect as in Strava? If not, then the difference is unavoidable. For setting the zones in Garmin see https://support.garmin.com/en-US/?faq=w8qlOr7BQ16Z82HVrevpw9