RoonBridge @ Rasp4 and MCh HDMI output

I use the Roon bridge with a NanoAvr HDA via LAN and HDMI which works OK for 2 Channel. I had to manually set the configuration to 1080p and 60hz for Volumio to play. There seems to be no setting to force 2-6-8 channel mode, and with Volumio alone it defaults to 2ch with no way to change that. True?

The RoonBridge is seen by Roon with three entries, where the one with the label “…Hdmi1“ seems best as it describes the HDMI port best (1).
However, the channel mapping is wrong and non consistent. Forced to 5.1 via Roon, it would only output 2 ch, and worse, sound distorted. When forced to 7.1, it would change rear and front and sometimes disable the centre.

Question: is there a way to configure the HDMI audio interface with Volumio and the Roon bridge? Like an EDID setting? Or even enable the second HDMI port for audio only (meaning with a very simple video stream for interleaving audio). Or even do channel mapping?