@Shanling
I tried to connect iPad Pro 12.9(2018) and Shanling M6(USB-DAC mode) with FiiO LT-TC1 OTG cable, and it worked!
But...
I noticed M6 was recognized as "M5s" by iPad Pro. I hope you to correct this in future firmware update.
I also noticed M6 was always charged by iPad Pro. I saw "Please try to avoid using this device when it is charging." on your Quick Start Guide.
Should I stop using M6 as USB-DAC with iPad Pro? If possible, could you add the setting to turn off charging on M6 when it works as USB-DAC?