I think there's a reasonable middle ground here. I would think it's fine for them to keep their algorithms closed-source, but be required to opensource the minimum amount of firmware to allow access to all of the hardware: bootloader, CPU, storage, memory, display, and sensors.

No need to reveal any trade secrets at all. The above stuff is not the secret sauce that makes people buy their watches anyway: they use some of the most basic hardware in the market at their price point.