I made this account just to tell you that i was having EXACT the same problem as you (same board, same RX).
I have the SBUS pad from R9MM connected to the SBUS pad on the F405 and S.Port on TX6.
Make sure you connect GND and 5V to the GND and 5V pads next to the SBUS pad = this solved my problem!!! I was feeding GND and 5V from another pad on the F405. I don’t think all de GND pads are 100% connected to each other.
The FrSky R9MM has an inverted SBUS signal, the SBUS pad (= UART1) on the F405 is also inverted (UART3 and UART6 are not).
Betaflight settings are:
PS: I’v" flashed the R9M and R9MM with ACCST firmware (not ACCESS) because ACCESS works only with FrSky radios. I have an Radiomaster TX16S. I don’t think this is relevant, but worth mentioning.