Re: Monitor is not detected at all, and ConnectedMonitor / UseDisplayDevice do not help.
I'm afraid that its not clear to me from your response whether the IIyama Vision Master Pro 17 is detected when connected to the VGA connector.
Please perform the following:
0) Attach the IIyama Vision Master Pro 17 to the (primary) VGA connector
1) As root, run:
mv /etc/X11/xorg.conf /etc/X11/xorg.conf.backup
startx -- -logverbose 5
And then attach the resulting bug report.