If you're tracking the shoes so that when you hit X mileage, you know to retire the shoes: when you put the shoe in Garmin, just change the "max distance" for the shoe to be X - whatever mileage the shoe had on Strava. Then you'll have your "retirement distance" for that shoe in Garmin.
Then when you get new shoes, just start them out in Garmin, and eventually they'll all be there.