Hi,
Presumably by “…. tried all restarts”., this includes a factory restore, is this correct?
If so, then as a DIY repair all you can do is to temporarily replace the display to check if the problem is on the systemboard or in the display.
Here’s a teardown [https://www.youtube.com/watch?v=YEmRnfd0kas|video] that shows how to replace the screen that may be of some help.
Search online for ''Samsung Galaxy S21 display assembly'' to get results for suppliers that suit you best.
***Note:*** ''Ensure that you get the correct one for your phone. Check the model number of the phone to be sure as there are several S21 models and they may not all be the same as far as the display assembly is concerned.''