It's all about building these things by yourself. The tools are there, and Pebble used to be the first smartwatch every open source diabetic I know used (including myself).
If you're not familiar, here's a good starting point:
https://androidaps.readthedocs.io/en/latest/
Oh, and you want to avoid Apple's hardware. Some of the software can only be distributed as source code, so installing them to an iOS device is not easy.
