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