I am an Apple Watch user for years - this device is awesome. The Ultra is even better but due to its size it is not very comfortable to wear at night, at least for me.
I wanted to track my sleep. I did this once over a year ago with my S5. To sleep with the Ultra was not an option for me. Beyond the size it's also the OS which is not very co-operative here as you have to enable sleep tracking manually by switching to the "Sleep" focus mode. The watch won't track your sleep automatically which I think is a shame in Apple's watchOS.
I found a very cheap (28 EUR) device on eBay marketplace, a Withings Activité / Pop hybrid watch which is small and light enough for the night to wear. It tracks the sleep very well, integrates superb with the Withings app and also syncs the data to Apple Health. However, the missing part is the blood oxygen measurement. The Apple Watch of course will track this data, but - you already know. Since I got the smart scale and also a blood pressure device from this company, I can tell this brand is very good, well crafted and easy to use.
Lately Xiaomi announced the latest Band 8 fitness tracker, and this brought me to have a closer look to this device. I found the "old" model on Amazon, the Band 7, for only 40 EUR and gave it a try.
This is a very smart device with a small but very nice OLED display. There is not much missing here: it tracks everything, can record workouts and forwards your phone's notifications if you want. I only need the sleep tracking function and the blood oxygen during my sleep. It also syncs with Apple Health.
You can't go wrong with this device. The silicon watch band however is not the most comfortable, but you can also get soft velcro bands which are perfect for the night to wear.
Resume: you do not need an expensive Apple Watch to track your health, you just get more options with it above the pure health measurements.