If you two didn't see, I have great news for you, there was a WWDC session after this post that provides a how to and example code to do exactly what I think you want to do!
Video: https://developers.apple.com/videos/play/wwdc2021/10009/
Example code : https://developer.apple.com/documentation/healthkit/workouts_and_activity_rings/build_a_workout_app_for_apple_watch