I tried with the GPU installed and removed all Boot Args except for agdpmod=pikera. That had the same result, Catalina doesn't load.
I tried with the GPU uninstalled and with Boot Args: dart=0 nv_disable=1 agdpmod=pikera. This time the Apple logo showed and the progress bar seem to get stuck half way, the machine did not restart. So I decided to restart it and try again in verbose mode. With verbose mode on, Catalina never loaded and the machine restarted.
Does starting in verbose mode ignore the Boot Args?
I tried again with the GPU not installed and with Boot Args: dart=0 nv_disable=1 agdpmod=pikera and verbose off, this time the machine restarted and never loaded the Catalina installer.
And then I tried again with the GPU not installed and with Boot Args: dart=0 nv_disable=1 agdpmod=pikera and verbose off... this time the Apple logo showed and the progress stopped about just under %50... I'm going to wait and see what happens, but as I'm writing this it has already been about 3 minutes and the bar has not moved.