Iz seems that the latest Windows 10 update KB5001391 solves most of the performance issues with FS2020 and I guess it was no a Asobo issue :-). All my stutters and flickers are gone since today even under heavy rain and heavy cloud densitiy with above settings.
Thank you very much for all your given support tips and hints and maybe see some of us somewhere in the sim world of FS2020 