I recently started migrating from 3.4.4 to 5.0.2 and started from scratch with openhabian and version 5.0.2. I have a running and working zigbee2mqtt installation an came across something I think that might be worth sharing.
If you install zigbee2mqtt via openhabian-config the configuration.yaml gets overridden by a zigbee2mqtt.env file in the data directory. This is not so good in my opinion, since it enforces the traditional frontend and enforces channel 26, which prevented all my battery powered ZigBee devices from working after restoring the backup.
So be aware to remove these overrides if not needed!