I have opened this thread because my information and questions are out of place in this thread.
The conversation regarding the problem Fairphone4 users face with a new device with StockRom (Android11) should be continued here.
I wrote an eMail to firstname.lastname@example.org after advice from @manoj, in which I described my problems and thoughts before the installation. As soon as I have a solution and a successful installation on my Fairphone4, I will comment here.
Of course, all opinions and questions about the topic are welcome.