Thanks, it was my fault after all. As I did mention earlier I =
had also file
called 10-monitor.conf and in that file I had already added section for
devices, that file was primarly loaded and overloaded settings from other
files.

I moved DRI settings to that file because I was sure it is being loaded:
...

Section "Device"
    Identifier "Device0"
    Driver "intel"
    Option "DRI" "3"
EndSection

Section "Device"
    Identifier "Device1"
    Driver "radeon"
    Option "DRI" "3"
EndSection

Now output of cat /var/log/Xorg.0.log | grep DRI:
[     5.164] (**) intel(0): Option "DRI" "3"
[     5.304] (**) RADEON(G0): Option "DRI" "3"
[     5.390] (II) glamor: EGL version 1.4 (DRI2):
[     5.394] (II) RADEON(G0): [DRI2] Setup complete
[     5.394] (II) RADEON(G0): [DRI2]   DRI driver: radeonsi
[     5.394] (II) RADEON(G0): [DRI2]   VDPAU driver: radeonsi
[     5.395] (**) RADEON(G0): DRI3 enabled
[     5.513] (II) intel(0): [DRI2] Setup complete
[     5.513] (II) intel(0): [DRI2]   DRI driver: i965
[     5.513] (II) intel(0): [DRI2]   VDPAU driver: va_gl
[     5.513] (II) intel(0): direct rendering: DRI2 DRI3 enabled
[     5.528] (II) GLX: Initialized DRI2 GL provider for screen 0

Checked problem:
1 not drawing content until resizing windowed mode applications with
DRI_PRIME=3D1 is solved,
2 did not check yet running fullscreen games
3 programs are still flickering but slightly different and much less,
4 did not check yet running fullscreen games
5 had no chance yet to plug in my external monitor yet

I will be doing some more tests later today and this weekend to see if other
problems are solved too, will write info asap. Big thanks for all help so f=
ar.