Hey, actually I didn’t investigate this any further. However, I found two other boards, which I havent tried out yet:
I have ordered the first one, however I also did not start development. The downside of this one is, it needs a proprietary firmware, where you need to get even a paid version if you need advanced features (power saving).
The second one I am unsure whether it can be used to implement end-devices (sensors). I have tried to post in that forum, but my post has not been approved for weeks.
The third board I have actually found is the Nordic Semiconductor based Adafruit Feather NRF52840 express (Adafruit Feather nRF52840 Express : ID 4062 : $24.95 : Adafruit Industries, Unique & fun DIY electronics and kits). As I understand it, it can be used to build zigbee end devices. However, it requires the use of the nordic semiconductor SDK and libraries and toolchain. Some months ago I have downloaded all the stuff, but never got to the point where I actually started development. There is no straight forward way of doing this, as you need to read tons of documentation, but the board itself looks awesome to me. Its tiny, and features a JST connector for connecting batteries, which can be recharged by connecting USB power to the board, which is great if you want to build battery powered devices.
All in all, I did not yet get to the point of a finished zigbee end devices, but this is what I have found out so far.
[edit] @Aleksandar-bit I have just quoted FloatingBoarders post as he referred to one of my topics. However adding to your actual question and topic, it might be worth mentioning that the board 2) I have posted (“MonaLisa”) is some self-designed board made by the guy in the linked forum, and its said to be suitable as zigbee router/coordinator. Maybe it would be worth contacting this guy for you. Maybe even if you dont want to use the board itself, the creator might have knowledge of the protocols and implementation and might help you out with your project.