I received my S21 ultra yesterday and have used my 10 years of best practice in order to maximise battery life whilst it's in learning mode.
When I smartswitch wirelessly:
1. I do NOT copy settings
2. I do NOT copy APPS
3. I do NOT copy homescreen
4. I allow the phone to fully drain out of the box and then fully charge when it's dead
My Samsung's have always come from Samsung directly and not from carriers. I do this to avoid the additional bloatware that carriers blast their handsets with.
I've enjoyed 8+ hours of Screen On Time and it just died a few minutes ago after 24 hours.
My current s21 Ultra settings:
Dolby - enabled
NFC - ON
Bluetooth - ON
Deep sleep apps - i've added the apps that i do not use often
Would you consider buying a handset from Samsung directly?
EE may have installed bloatware on your handset that could be causing this issue.