I tried GadgetBridge because it cannot sync the activity files (.fit and/or .gpx) so I still had to plug the watch into a computer to keep the actual data.
So I ended installing ActivityLog2[0] to do something with the files I had to have on desktop and GadgetBridge was of little use because relying on GadgetBridge without actually syncing the files might make me forget about doing the backup to a device I control (GrapheneOS or a computer).
As soon as GadgetBridge support syncing the files from the watch to the app (or any local folder on Android), I'll install it again and stop doing the manual backups over USB. Syncthing will do it automatically.
Under settings->automations->auto export, you have "Auto export zip" where you can specify export interval. The zip file includes all the data (personally, I only see .fit files) from your app. For sync, you might have to use something like syncthing.