I've actually done extensive work with reverse engineering garmin's bluetooth protocol for setting the time on my vivofit 3 watch without making an account!

It's mostly shambled together stuff but I have a wireshark plugin that works pretty good for the few pieces it supports

https://github.com/mjsir911/GarminBLE

I started this project with no bluetooth experience either, it's definitely been a lot of learning.

Let me know if you are interested in continuing / working on this, might give me a nudge to get back on it.