I'm interested in a device that can measure respiratory rate, but so far Apple only offers a breathing app that does nothing more than just tell the user how to breathe, i.e. without any feedback based on actual respiration.
https://support.apple.com/guide/watch/breathe-apd371dfe3d7/w...
https://9to5mac.com/2020/01/06/apple-watch-breathe-misconcep...