

I don’t want a PC and all the attendant complexity
Not all that much complexity these days, unless you wanna do it on Linux - but even that’s getting easier and easier.
Mostly you just start Steam, install the game you want, and launch it. Done. EVEN on Linux these days for most games.



You may want to look at the supported devices on this project. It’s an app to get data to/from wearables.
Two of the supported devices are bangle.js and bangle.js 2. These are open source smart watches with GNSS and HR, which have different apps available already that I believe can record data without your phone connected, and then sync later, either to your phone or computer (not sure on that one). And you can develop your own apps too of course.
I don’t really know much more than that. I do want to get back into exercising regularly, and would like to track my progress, so I may get one myself at some point.