Maybe I'm missing something but it seems impossible to be able to create custom Activity apps because the ActivityRecording API doesn't allow you to insert any data into the Activity FIT file. You can record a session and it will get HR, distance, time, calories automatically just like the built-in apps do but you can't add anything other than a custom field in the header or add a lap. So this might work for running, walking types of apps but it's useless for anything else.
To give an example, I'm doing the 100 push-ups / day challenge so randomly during the day I do a set of push-ups. I could use the Strength Training app for that but then I'd get a new Activity for every set so my timeline would be littered with many each day. What I want is an app to keep track of each set and then roll them up into one Activity at the end of the day. I've written a simple app to record each set using the Storage api and that works fine, so I can keep track of how many I've done. Now for the hard part, how can I create an activity and add each set into it ? I'm really not bothered about HR, seconds, calories etc, but numbers of reps is kind of essential.
Searching these forums, reveals that many people have hit a blank with this and many related tasks, e.g. creating swimming activities.
GARMIN: please advise if there's a way of doing this or please indicate a timeline for providing a more general purpose API. It seems at the moment, having the most sophisticated sportswatch on the market is actually useless for custom sports and I'd actually be better with something that has better general API's like WearOS or Apple (yuck).