Smart watch:

ZsWatch. They are developing the PPG sensor and I have used their PPG sensor hub driver for my own smart watch project. The guy heading it up is super knowledgable.

Healthy Pi Move this one seems interesting but maybe less open.

Also with GadgerBridge, you can have complete local control of many smart watches like Amazfit (Helio strap is my choice there for accuracy and whoop alternative. Bip3 is also very accurate) and a lot of Garmins and xiaomi.