Hi Argiri,
I am not sure if you found the solution but check out Eventflags (EventFlags - API references and tutorials | Mbed OS 6 Documentation) for sleeping in system on mode.
I am using a custom nrf52832 device sleeps at 30μΑ (it is not consistent, I am still trying to figure out why, it doesn’t sleep in every cycle) (mbed os 6.14)
Kind regards,
Rafaella