I tried and had some success about 5 years ago on a Fenix 2. Protocol got encrypted afterwards but looks to be still similar (I got the Fenix 3 to "work" at some point). In any case, not an easy task by any means.

The sync protocol mostly just sends all the files to the Garmin server so there is not a lot of logic locally. I no longer have any interest in the topic but if I were to restart, I'd focus on the "Explore" protocol which seems to allow upload/download of waypoints/tracks which is all that I wanted from the device in the first place.