Here is an article that compares charging times on the newest vs somewhat old apple watches: https://www.androidauthority.com/how-long-does-apple-watch-t...

Also Apple introduced a "Low Power Mode" on WatchOS 9 that allows you to potentially get 60 hrs on the Apple watch ultra. This link discussed what is disabled in this mode. Seems like it would work for your use case or at the very least you run low power mode during the day and turn it off at night for better sleep monitoring: https://www.macrumors.com/how-to/enable-low-power-mode-apple...