Thank you for the suggestions and support.
After a complete day of experiments I found hat rendering it as is (the suggested Defer Atmo ON) is the best way.
I found that there are no time savings in setting it to OFF. To get an adaequad quality like Defer Atmo ON I had to increase Micropoly Details so high that the rendertime was the same but some rest noise still remained. If I took Oshyan's warning of possible flicker into account, my decision was probably right
Just examples from yesterday:
No Defer Atmo
Micropoly Detail 0.3
AA 5
1920 x 1080 cropped to an Area of 544 x 340
Rendertime 17 minutes
[attachimg=1]
No Defer Atmo
Micropoly Detail 0.8
AA 5
1920 x 1080 cropped to an Area of 544 x 340
Rendertime 96 minutes
[attachimg=2]
Defer Atmo ON
Micropoly Detail 0.3
AA 5
Complete frame rendered (1280 x 720)
Rendertime 105 minutes
[attach=3]
I should meniton that I have the impression that rendertime is very variing, depending on the cropped area. I often had the effect that I have very huge differences in render time if I move the cropped area only some pixels... especially in my most frightening renders: in the face of sun and a dense atmosphere and "Receive shadows from surfaces"
... little addition: still working on 4.1.24 because my maintenance days expired. I'd like to wait until some great updates for my personal interest (sorry may sound a bit egoistic
) These were eg. grouping objects, handle lghtsources like objects and group them with objects and stuff like this. But let's see what is coming next I fully trust Matt's and Oshyan's phantasy and talent