I became aware of the Smart Band concept, but most of them do not support the Home Assistant Companion App (which is on WearOS watches only), or app development at all. The brand that I found that allows app development on their smart bands/watches is Zepp Health through their wearable operating system called ZeppOS. The company owns the Amazfit brand which has a line of wearable products that use ZeppOS, but also some other wearables have ZeppOS, such as the Xiaomi Mi Band 7.