I don’t know if this is directly helpful for your project, but the link below has a sketch. My project needed to sleep for a certain amount of time, so I used the delay() function to enter SystemOnSleep. In this non-mbed BSP, just write delay() and it will go into SystemOnSleep.
‘Sleep Current of XIAO nRF52840, Deep Sleep vs. Light Sleep’
‘System_ON_Sleep of XIAO BLE - #63 by msfujino’
edit
I wrote a sketch for Wakeup with GPIO interrupt. Sleep current is about 25uA.
nRF52_XIAO_LighSleep_InterruptWakeup.zip (1.4 KB)