Thank you for the suggestion, I will try!
Thank you for your answer! 
I am not sure if this is could be a problem related to freesync because other Distributions like OpenSuse Tumbleweed of MX Linux 21 don’t show any flickering with the same hardware setup.
I can lower the refresh rate to 120Hz to cease the flickering. However, after returning to 144Hz the flickering returns too.
I have disabled the compositor under Xfce as you have suggested and the flickering seems to be gone.
Can my problem be related to the compositor? 
But why can I use OpenSuse Tumbleweed KDE with compositor and no flickering? Strange.
I have used the Snapshot from Aug (offline Installation with -Syu afterwards) to test this. The errors in the Xorg.0.log relatated to Amdgpu did not appear here (but the flickering does, at least with 144Hz and active compositor).
As OpenSuse Tumbleweed KDE has no flickering with KWin Compositor in use, I will test KDE aswell with the Snapshot from August as the installation from Atlantis neo has flickering with KDE and Compositor (but also has the amdgpu-errors).
I will report back!
It would seem so. I suggest giving picom a try. By adding it to startup programs it can composite xfce desktop. Also you can tweak settings via its config file.
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maybe this can help you:
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I will try for sure! Thank you for your suggestion!
For the moment I have installed the KDE desktop from the August Snapshot. The flickering is still present.
However, the Xorg.0.log has also changed a bit. This one still persists:
5.800] (EE) AMDGPU(0): Failed to make import prime FD as pixmap: 22
And the others have changed to:
[ 5.949] (II) config/udev: Adding input device HDA ATI HDMI HDMI/DP,pcm=3 (/dev/input/event12)
[ 5.949] (II) No input driver specified, ignoring this device.
[ 5.949] (II) This device may have been added with another device file.
[ 5.950] (II) config/udev: Adding input device HDA ATI HDMI HDMI/DP,pcm=7 (/dev/input/event13)
[ 5.950] (II) No input driver specified, ignoring this device.
[ 5.950] (II) This device may have been added with another device file.
[ 5.950] (II) config/udev: Adding input device HDA ATI HDMI HDMI/DP,pcm=8 (/dev/input/event14)
[ 5.950] (II) No input driver specified, ignoring this device.
[ 5.950] (II) This device may have been added with another device file.
[ 5.950] (II) config/udev: Adding input device HDA ATI HDMI HDMI/DP,pcm=9 (/dev/input/event15)
[ 5.950] (II) No input driver specified, ignoring this device.
[ 5.950] (II) This device may have been added with another device file.
[ 5.951] (II) config/udev: Adding input device HDA ATI HDMI HDMI/DP,pcm=10 (/dev/input/event16)
[ 5.951] (II) No input driver specified, ignoring this device.
[ 5.951] (II) This device may have been added with another device file.
[ 5.951] (II) config/udev: Adding input device HDA ATI HDMI HDMI/DP,pcm=11 (/dev/input/event17)
[ 5.951] (II) No input driver specified, ignoring this device.
[ 5.951] (II) This device may have been added with another device file.
This looks very interesting! Thank you!
However - Can I use this as it is or do I have to modify it because I use amdgpu as driver and not radeon?
you can use it as it is
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This leads to an unbootable System. I can access the TTY but with “startx” I get the warning, that no monitor has been found.
What about this instead:
Creation of the file “20-amdgpu.conf” in /usr/shareX11/xorg.conf.d with the following entry:
Section "Device"
Identifier "AMD Graphics"
Driver "amdgpu"
Option "TearFree" "on"
EndSection
Just the basics before I try out any options. At least this should match my GPU-driver I guess?
Edit: Correction of the path for the file
Edit 2: Correction of the TearFree Parameter
No harm done, no foul
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Oh, I don’t blame you I am happy that you have tried to help me here. Thank you for this. 
Not sure if this is helpful or not … but I am using EndeavourOS/Gnome/Wayland on an AMD 5700G with a 144h refresh and no flickering … do you have the option to run KDE with Wayland? Might be worth a try if you can?
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Thank you, I will try this as soon as possible! I guess I will change to Gnome then for this test to match your configuration as close as possible and report back!
Before I tried out wayland I have created the 20-amdgpu.conf - file as mentioned before and made a reboot.
So far the flickering seems to be gone. I will observe it further and mark this as solved if it turnes out to be true after some hours of testing.
@darknetmatrix
Thank you very much for your help here!
One question for you however:
I had to modify your file a bit to make it work for me. You have listed two more lines in your config for radeon.
Option "DRI" "3"
Option "AccelMethod" "glamor"
Do you know if these one will also work with amdgpu or are they restricted to radeon only?
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I think that they will work with amdgpu, can not test it myself because i have radeon.
Something like this maybe?
Section "Device"
Identifier "AMDGPU"
Driver "amdgpu"
Option "AccelMethod" "glamor"
Option "DRI" "3"
EndSection
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I will try out thank you!
One more question, sorry: 
In my /usr/share/X11/xorg.conf.d there are now two files regarding my driver:
10-amdgpu.conf with the content:
Section "OutputClass"
Identifier "AMDgpu"
MatchDriver "amdgpu"
Driver "amdgpu"
EndSection
And the newly created 20-amdgpu.conf with
Section "Device"
Identifier "AMD Graphics"
Driver "amdgpu"
Option "TearFree" "on"
EndSection
Is the first one obsolete now and can/should be deleted or can I keep it as it does not matter?
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on both my systems I created the 20-radeon.conf the 10-radeon.conf i have left
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Perfect, thanks!
Here is some more info for you.
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Thank you, this is very helpful!
According to this I do not need to add those two lines because they are already set to suggested values by default?
Or are these two options only active if they are listed in the config file?
Not sure what you read? I think it’s saying for the configuration details that if you set any then those are the defaults. Such as these. I’m sure if there is no configuration then no it’s not enabled.
Option “TearFree” " boolean "
Set the default value of the per-output ‘TearFree’ property, which controls tearing prevention using the hardware page flipping mechanism. TearFree is on for any CRTC associated with one or more outputs with TearFree on. Two separate scanout buffers need to be allocated for each CRTC with TearFree on. If this option is set, the default value of the property is ‘on’ or ‘off’ accordingly. If this option isn’t set, the default value of the property is auto, which means that TearFree is on for rotated outputs, outputs with RandR transforms applied and for RandR 1.4 secondary outputs, otherwise off.
Option “VariableRefresh” " boolean "
Enables support for enabling variable refresh on the Screen’s CRTCs when an suitable application is flipping via the Present extension.
The default is off.