Not for Garmin, but I have saved my data from the servers for the Microsoft Band [https://jeffhuang.com/extracting_my_data_from_the_microsoft_...] and Hello Sense [https://jeffhuang.com/extracting_my_data_from_the_hello_sens...] before they both shut down. Basically, you can MITM to find the REST API calls, save the token, and mimic the calls to get your data.

I was thinking of one day editing the apps for wearables to direct them my own server, and then setting up a basic CRUD-like endpoint server to keep discontinued cloud-based wearables from becoming garbage by allowing them to have basic functionality. But haven't had the time, and would have to look into the legality of it first.