Do you have any sensors (e.g. wheel speed, cadence, power meter) on your individual bikes? If so, you can use https://www.activityfix.com/user_cp to create automatic rules for assigning your rides to the correct bike in your Strava Gear.

You can't edit the start mileage on a bike, but you can create a manual activity with the prior mileage and assign a bike to that.

https://www.probikegarage.com/ is an awesome app that can help keep track of things too, including individual components. I believe that you can assign a start mileage with the app.

How big is the mismatch between Garmin and Strava for your last 13 months? The way Strava interprets Garmin data can lead to ~.01 or maybe .05 miles of difference per activity, and that can add up.