Another alternative I found is @RIOT_OS.

I face here the same questions as for the Zephyr RTOS:

- can IEEE 802.15.4 be freely used without any membership fees,

- how much of application level and API(s) needs to be custom-developed on each side (device and integrations).

https://doc.riot-os.org/group__net__ieee802154.html

I found @zephyr as an alternative, but I'm not sure whether the IEEE 802.15.4 can be freely used without any membership fees.

And, that's also quite low level/layer protocol. So, I guess, that application level integrations would need be custom developed.

Where Zigbee and Z-Wave standards define also application layer and API(s), and devices can be self-describing (at least in Z-Wave, not sure about Zigbee).

https://docs.zephyrproject.org/latest/connectivity/networking/api/ieee802154.html