I’ll continue to check what I can on the hardware side, but this looks very similar to Mmc2: error -110 whilst initialising SDIO card - #19 by ajudge.
While the MM6108 can support a 1.8V I/O voltage for the SDIO lines, the RESET line needs to operate in the “VBAT” domain, 3.3V.
Unfortunately at this stage the solution may require a pull-up resistor to be inserted on the RESET line to 3.3V, as well as some device tree adjustment to configure the SD2_RESET pin as an open drain GPIO, and finally adding an mm6108 specific child node in the usdhc2 controller to provide a reset-gpios to the Morse driver so it can control that GPIO.
Are you in a position where you can add a hardware pullup to that RESET line? Does the bootloader allow you to load an alternative device tree binary?