Welltory: Heart Rate Monitor works with your Apple Watch and the Health app to get comprehensive data about your heart rate and stress. It gives you daily health metrics on a wide variety of insights about your overall health. In all, Welltory tracks over 140 data sources to bring you detailed info about your health, exercise regimen, and recovery after your workouts. You can also monitor your progress over time, informing how much you get out of your various exercises. It supports several other devices, in case you use Garmin, Fitbit, Oura, or other trackers.
As far as relieving stress is concerned, Welltory has you covered with a breathing app for calming your mind in those instances when life and your day-to-day can feel overwhelming. It offers daily life hacks to improve your health and ensure you allow yourself time in the moment to heal. The app also has a subscription-based service to give you more info, but even without it, you get a lot of information, and it doesn't constantly bug you to upgrade.