2/21/21 UPDATE: I gave my SP7 to my brother and have since purchased two more SP7s. The second is a 10th gen i7 (16 ram, 256 ssd) and the third is a 10th gen i5 (8 ram, 256 ssd). Both were purchased open-box from Best Buy and based on the manufacturing date were around 10-12 weeks old.
In tablet mode, the i7 gets around 7.5 hours of continuous YouTube viewing and the i5 gets 8.5 hours of continuous YouTube viewing (50% brightness, power mode on balance - best battery life, Wifi and Bluetooth on, and keyboard cover detached). The only new thing I've been doing is testing the device's battery life after a non-security Windows update and if the battery life decreases, I uninstall the update immediately.
This is definitely not ideal and one should not have to do such things to maintain decent battery life, especially since my 2020 15-inch HP Spectre x360's (11th gen i7, 16 ram, 512 ssd) battery life is not impacted by any Windows updates and I get around 12.5 hours of continuous YouTube viewing on it (4k resolution, 50% brightness, power mode on balance - best battery life, Wifi and Bluetooth on).