Previous US case law is pretty clear on disasembling and understanding hardware you already own, it's legal is fun and good for your soul. See DMCA Section 1201(f).
On the brightside of things, Garmin has historically been largely open with reverse engineering. Here are some examples:
Garmin's FIT file format was reverse engineered extensively by the community for years. Projects like PyGarmin implement the protocol used by Garmin GPS devices based on the official protocol specification — Garmin eventually published the spec themselves. They now maintain official open source FIT SDKs. They chose to embrace it.
Atredis Partners reverse engineered the Garmin Forerunner 235 firmware and virtual machine completely, publishing a detailed blog post about it. The researcher noted that Garmin was easy to work with while reporting issues — explicitly saying this wasn't an indictment of their products. Garmin did not sue them. They cooperated on vulnerability disclosure.
A Wireshark plugin for Garmin's BLE protocol has been publicly available on GitHub for years with no legal action.
A comprehensive Python3 API wrapper for Garmin Connect providing access to health, fitness, and device data has hundreds of stars on GitHub, has been active for years, and Garmin has never touched it legally despite it reverse engineering their Connect API.
Our project seeks to understand and improve the product we already own. We do not seek to reproduce and create items that compete with existing Garmin products.