I am 90% sure that your battery drain is caused by the mediocre 4G signal which is used for internet at all times. A lot of power is basically being allocated to get a better signal and thus while you use WS all the time, your batt drains FAST. If you happen to have Wi-Fi for a day or two and the signal is maxed out, the battery will surely last longer. Generally speaking though, you cannot compare other peoples SOT to yours as most people do not use WS as much as you, nor 4G at all times.
There might be some other extra settings that could be altered to improve battery, like:
adaptive battery ON
enhanced processing OFF
location OFF
background data OFF
resolution to FHD
background usage limit to put rarely used apps to sleep
NFC OFF
nearby device scanning OFF
And of course the last version of android 11, as it seems A12 is not yet worthy or ruling the S21U. Sadly I have a feeling that Samsung might not fix A12 for the S21U as they are now releasing the S22. Hope I'm wrong.