I’m using the plain Xiao nrf52840.
I found it easier to optimize for memory size with Zephyr since it is highly configurable. I found it harder to optimize for power with Zephyr because of the abstraction and indirection. My target application is using Zephyr on a nrf52810 which I needed to optimized for size. I experimented with arduino to help figure out baseline power.
I was able to get 1uA system_off sleep and 15uA system_on idle with unmodified arduino. With modified arduino interrupt code I was able to get 3uA system_on. The code is posted here Getting lower power consumption on Seeed XIAO nRF52840. The topic also has links to other pages on nrf52 power optimization.