How can I disable AMD radeon and use Intel Integrated Graphics on 2011 iMac?
There is a Gnome shell extension to select the used graphics device from System76. The extension can be used with other desktops than Gnome as well (only from the terminal). This extention us usually used to save power, but it might be useful for your problem as well.
System76 gnome shell extension installation guide
This is not a very clean solution but might solve your problem after blacklisting was not successful.