Unfortunately, watchOS 10 doesn't offer granular control over background app activity like newer versions do. However, you can try a few workarounds to help minimize the impact of background app activity on your battery life.
One approach is to disable Background App Refresh for certain apps on your iPhone that are mirrored on your Apple Watch. By doing this on your iPhone, you may indirectly reduce the background activity of those apps on your Apple Watch as well.
To disable Background App Refresh on your iPhone:
1. Open the Settings app on your iPhone.
2. Scroll down and tap on "General."
3. Tap on "Background App Refresh."
4. Here, you can either toggle off Background App Refresh for all apps or selectively disable it for specific apps by toggling them off individually.
Since watchOS 10 doesn't offer direct control over background app activity, this method might help mitigate the issue to some extent. Additionally, you can also try minimizing the number of apps installed on your Apple Watch to further reduce background activity and preserve battery life.
If these steps don't provide the desired results, reaching out to Official Apple Support for personalized assistance tailored to your specific device and software version might be the next best step. They can offer further guidance and troubleshooting tailored to your Apple Watch Series 5 running watchOS 10.