Hi there,
Yes, SO Nrf_sdk has a defined Nrf52840 SOC DTS file. The Zephyr mainline board repo doesn’t have a Seeed Xiao in it so They try and use the nrf DTS and it ofcourse causes issue’s. YOU can West command your way to installing , Building and flashing just with west commands, and say Menuconfig is all you would need. Its a chicken & egg Situation.
Zephyr is the underlaying layer, Nrf_sdk is on top of that. However the underlayer relies on the upper layer for the board support, You see that makes it appear AFU’d That is to say You need to have the proper West – Version and paths very straight before you get fancy with multi-image DFU moves 
The future is MCUboot +app+DFU (serial, BLE, webpage)
here is why,
- Zephyr Bluetooth LE Controller
- This is a generic Bluetooth Low Energy (BLE) controller that comes with Zephyr OS.
- It is not optimized specifically for Nordic devices but is available as an alternative.
- It works across multiple platforms, including some Nordic chips, but lacks Nordic’s proprietary optimizations.
2. SoftDevice Controller (Recommended for nRF52, nRF53, nRF54)
- This is Nordic’s official BLE controller, developed and optimized specifically for Nordic’s nRF series.
- It provides better performance, efficiency, and power consumption for Nordic chips.
- It is thoroughly tested and supported by Nordic Semiconductor.
Why Nordic Recommends SoftDevice Controller?
- It is optimized for power efficiency and performance on Nordic devices.
- It has better support for Nordic-specific features like advertising extensions, long-range mode, and high-duty cycle scanning.
- It is actively maintained and tested by Nordic, ensuring better stability and reliability.
Nordic has it on lock so just keep going there’s is all I ever needed, does everything and more, and on the Nrf54L15 Silicon it’s a GAME changer.! Period.
The other thing is getting the partitions correct for the MCUBoot to work and be DFU’ Ninja 
HTH
GL 
PJ