So I had put my problem into this thread: [Stable Update] 2020-11-22 - Kernels, Pamac, Gnome, Jade, OnlyOffice, Xorg-Server - #20 by philm
And it was split by @Yochanan into a new thread: Screen brightness issues
In this new thread a solution was offered that did work for me!
Hopefully this fix will work for others as well! Many thanks to @Jack1221 
!
[EDIT] And yes, this solution works for the 5.9.10-1 kernel.