On device only, no play store, with GrapheneOS.
I am thinking about a Garmin watch and using it with Pulse. My cardio is really bad and it has been starting to cause all sorts of health issues, so I need something to track my heart rate while I go on walks to build my stamina back up so I can start actually working out.
Gadgetbridge supports a handful of heart rate monitors as well as many smart watches if you want to look at more than just Garmin. You could likely use the Garmin with Gadgetbridge too.