If you want just workout/health data, there is a way that is supposed to work with iTunes, though I have never tried it. Using the app "Decipher Phone Refresh", you make an encrypted backup with iTunes on your computer. Then you run Decipher Phone Refresh and follow the prompts to allow it to create a version of the backup that has only health data, and then restore that to the phone using iTunes. It will only overwrite that data on the phone, nothing else. (The app walks you through all of the steps.)
Workout data is really not stored on the watch, as far as I can tell, at least in a way that is exposed to the user. The data syncs to the Health data on your phone, and you can look at past workouts in the Activity app on your phone, not on the watch. (Though the workout app does remember which workout you did last in each category and adds a "Quick Start" for that workout - I'm not sure if the watch will get that when you restore from Decipher Phone Refresh.) So, this may do what you want, at least as much as can be done on a phone on which you don't do a full restore. What I'm not sure about are the achievements in the Activity app.
So, again, I have never tried it, but others have. And it's not iCloud. I do wish that Apple would add that functionality - make health data something that is synced with iCloud from the phone, or at least make it an option for those who wish to risk having health data stored outside the phone.
See
https://deciphertools.com/decipher-phone-refresh.html
See also
https://forums.imore.com/ios-10/3914...ml#post2965156
By the way, I have tried the app mentioned in one of the posts in that thread (Health Data Importer) and it did not work the way I wanted to - workout and activity data did not show up in the activity app, no achievements, etc. Saying that, I have not use Decipher Phone Refresh either, but it may be worth a try? (It's free for health and activity data restores.)
