My dad got one shipped (imported) to germany for less than 50Eur. The 3 issues he had was that the notifications didn't work on his phone (an android update fixed that), random disconnects on some phone models and that the current alarm app only supports one alarm.
I'd advise against pinetime as a sports watch because it doesn't accurately track mileage or pace. I'm looking to replace mine eventually with a Garmin or something.
Well, you can look at a gadgetbridge-compatible watch. gadgetbridge is a foss android app that replaces the manufacturer's app for snartwatches, earbuds, and a few other things. it's a good starting point, but most of the watches aren't directly foss. if you want a watch like that, you have two options. the pine64 pinetime is open source, but not easy to modify if you want a sealed, water resistant version. it also hasn't got the best specs. the bangle.js is a "hackable" smart watch, which has a a SWD interface on the outside, making it an easier option to modify. I can't say as much about it's fitness tracking, but it does have a heart rate monitor and accelerometer.
Can you be more specific? Are you talking just on mobile?
For watches and such on mobile, the device maker has to write software that runs on the watch, but interacts with the mobile OS's API in order to display information. It connects to your phone as a subscription device to things like alarms, messages, notifications and calendars. So if whatever app you're using on your phone routes events through these systems, they will work with your software.
If you're talking about integrations with desktop software, I'm unaware of any watch devices or device OS's interact with anything like that.