Garmin to Xert and Xert to Strava. This will add Xert info to your entries on Strava. Drop Garmin to Strava. Doubles are dealt with adequately by both Strava and Xert, but the extra info you get from Xert, including a Google maps image of the spot of your BT, if that happened outdoors, is just cool.
Mainly because it’s a two-way sync, whereas Garmin is (always) one way. So, Xert can only write their additional metrics to Strava, which makes it more interesting.