Reading the forums, seems like Deconz is deprecated vs. ZHA, yes?
Using ZHA, I can add devices easily enough, and it has a visualization feature.
Is there any reason to also use Zigbee2MQTT if I don’t need MQTT?
I use zwave2MQTT because Zwave JS seems more limited; zwave2MQTT adds more features.
Not sure Zigbee2MQTT does that much more than ZHA by itself?
Where did you read that? That’s definitely not true. I’m happily using deconz for over 5 years now.
You can not use ZigBee2MQTT in addition, you can only use one ZigBee integration at a time, so it’s either ZHA or ZigBee2MQTT.
There are countless topics here discussing which one is the best. If you are happy with ZHA, just stick with it.