You did not tell which Garmin device you own, but if you do not find the instructions for the reset to factory defaults in the user manual for your device, the following article should work minimally for most of the outdoor watches. See the section "Delete Data and Reset Settings" or "Unprompted Reset"
That is necessary to get the data and the metrics from the watch. Data that was already synced, is more difficult to get rid of. As for the activities - you can delete them either manually, or by a script (search the forum - there are some older threads showing examples). If it is a new Connect account, the simplest would be discarding it, and creating a new blank one.
For example the following thread may help, but there are others, on the forum, as well:
As for the remaining data, like the historical values of VO₂max, LT, HRV, Training Load, etc - creating a new Garmin Connect account is probably the only way to assure getting rid of all of it.
