I have found an excellent Arduino package for Adafruit nRF52840 development boards with OpenThread support: soburi/openthread_nrf52_arduino
To use it, you have to burn a new bootloader on your board. You definitely have to use a J-Link programmer, it is not possible to do that with uf2 files and the uf2 flash disk. Ask how I know that. Yep, I bricked my ItsyBitsy.
So I bought a J-Link programmer (clone), soldered two wires on the SWDIO and SWCLK pads on the underside of the ItsyBitsy and reanimated my board as described by soburi. It is now ready for OpenThread in the Arduino IDE.
Based on the example I wrote a sketch (ot-sensor.ino) which reads out a MS8607 sensor and sends the data via IPv6 multicast to the OTBR. At startup the red LED lights up for 2s. Press the SW-Button on the board until the light goes out. The board is now in joining/commissioning mode. Once joined, it will connect at once to the thread network after start up.
Based on an example on Home Smart Mesh, I wrote a litte python script which listens on the UDP port and publishes the data to MQTT: thread2mqtt.py
Now subscribe to the topic:
stl@atom:~$ mosquitto_sub -h localhost -t thread/
{ CurrentTemperature: 23.12, CurrentRelativeHumidity:29.79, CurrentPressure: 969.03, Alive: 2879 }
{ CurrentTemperature: 23.12, CurrentRelativeHumidity:29.85, CurrentPressure: 969.04, Alive: 2880 }
{ CurrentTemperature: 23.12, CurrentRelativeHumidity:29.82, CurrentPressure: 968.98, Alive: 2881 }