Activities are stored in the FIT format, which is publicly documented and they have a rather easy to use sdk available for it as well[0]. Its fairly trivial to get other daily metrics out of the watch as well using Connect IQ, you can even write out to a plain text log file if you want.
If you dont like how they implemented it you could easily write your own, the api is fairly straightforward: https://developer.garmin.com/downloads/connect-iq/monkey-c/d...