This was my abbreviated setup checklist
Run and have access to OTBR WEB
Correctly recognize RCP dongle
Autodiscover Thread after enabling the OTBR integration http://localhost:8081
Have HA create a new preferred thread network & enable “used for Android + IOS credentials”
Run python-matter-server and enable matter integration in HA ws://localhost:5580/ws
Sync Thread Credentials on mobile device
Settings > Companion App > Troubleshooting > Sync Thread credentials
Run sync twice, the message should read “Home Assistant and this device use the same network”
Device Pairing
Add device via HA Companion App Settings > Devices & services > Devices tab > Add Device button > Add Matter device