I would highly consider leveraging the AsteroidOS project – a privacy-focused linux smart watch effort – on one of their approved devices. That link should bring you straight to the watches they support.
Pine Time works well with Pine Phone, but only has basic functionality with other Android devices, like notifications. Not much else last I looked, but I may be out of sync with the community’s development efforts.
The Bangle.js 2 smart watch is another open source device you could look into.