I looked into it but watches, like phones, fall into the category of things that really do just need built by a company with tiny unsolderable-by-humans parts.

I looked into it but watches, like phones, fall into the category of things that really do just need built by a company with tiny unsolderable-by-humans parts. When I checked maybe a year ago I found a fitness tracker that someone had found a way to upload new firmware to, but the hardware was no longer manufactured so you had to find second-hand parts on eBay, and there weren't many on sale in Europe. I gave up hoping the idea of an open hardware health watch would be the topic of a Kickstarter in the future.