I also have an R9MM on SERIAL6 with the same serial params as you. When you say you get GPS, RSSI and RX voltage, do you mean on an OpenTX telemetry screen on the Taranis? The telemetry sent from ArduCopter on T6 back to the Taranis over FrSky passthrough would typically be displayed with an app such as yaapu - is that what you’re using?
With SERIAL6_OPTIONS=7 I have T6 connected to the regular S.Port pin on the R9MM (the one next to the ground pin).
My current and voltage sensing calibration parameters are different than yours. I copied mine from here.
I have RSSI_TYPE set to 2 so I can send RSSI over an RC channel and display it on the OSD. I notice you have OSD1_RSSI_EN=1 and RSSI_TYPE=0 (disabled) - do you get RSSI on the OSD?
I’m using a BN-880 mag/gps. I’ve used several of these and have mostly been happy with them. I have never changed any compass parameters in ArduCopter - 3.7dev on a Kakute F7 always seems to just do the right thing automatically.
Here is my param file.