@Fiona26 Glad you are still impressed with the S21 Ultra. I also have a few routines for Brightness and Power Saving at certain times of the day. Personally do not use Dark Mode much though. Are you using the Natural or Vivid Mode? I am taking it that you do not use some social media apps a great deal.
I do not work for Samsung or make Samsung Products but provide independent advice and valuable contributions.
My Device- S21 Ultra 5G ( SM-998B/DS ) CSC= BTU , 12GB/256GB
One Ui 4.0,Android 12 Official Version.