As I mentioned in the project details, I spent a lot of time trying to get my XBee to communicate with Zigbee2MQTT, so I'm going to discuss that a bit here.

To get this to work, I had to manually configure the XBee to initiate itself with z2m.  Specifically, I used XCTU to configure the XBee module.  This guide that I stumbled upon was immensely helpful in helping me understand the data frames that I needed to send to z2m to add my device.  If you follow that guide exactly, you will add your device as a simple on/off switch (0x0002). I wanted to add my xbee as a color light, so I used a device ID of 0x0102.  This PDF, although not about the XBee, was very helpful for me to figure out the device IDs and clusters.

Once I finally had my XBee added to z2m, it showed up as an unsupported device. To fix this, I had to do two things in z2m: add an external converter and modify the database.db file.  To add an external converter, I followed the guidance here and here.

Finally, even with an external converter, z2m doesn't know to use that external converter because z2m does not add a modelID to the database file. I simply edited my database.db file to add "modelId":"XBEE3" to the entry for my xbee device. You need to make sure that whatever you add for modelId is the same as what you used for zigbeeModel: ['XBEE3'] in your external converter.

I tried to add a "device response" handler to my external converter, but I could never figure out what data z2m was expecting to receive back from the device.  Ultimately, I removed the device response code.  I would definitely appreciate some help here if anyone has ideas.

In the future, I would incorporate these steps into the power-on sequence for the arduino.  Right now if you wanted to implement this project, you would need to manually add your XBee first before running this code, which is less than ideal.