I'm not the developer, but there is now a subsonic client for music-enabled Garmin watches.
See here: https://apps.garmin.com/en-US/apps/600b ... 4fcfdcf794
and here: https://github.com/memen45/SubMusic
It's in the early stages of development and seems to work pretty well. Your server will need to have either http access or, if https only, will need a legitimate certificate and not the subsonic self-signed one.