Due to the size of my UPB deployment (several dozen line devices), I set up a separate WiFI network on its own subnet and VLAN. Each device uses a static IP on that subnet matching its UPB device ID (that is, UPB device 12 uses IP address my.sub.net.12.) I found it much easier to keep the static IP to device mapping than to mess with DHCP; this way if you have connectivity issues you can just ping your.sub.net.deviceid and know you should be getting that target.

For smaller deployment or to start with, you could just assign your devices and bridge static IP's on your existing subnet. Also unless you are a network admin extraordinaire (I am not!) I do not recommend VLAN's.

I use an RPi for MQTT because I had an RPi running other stuff, so it was no additional hassle. There is an MQTT broker for ESP32, but it uses ESP-IDF, not the Arduino IDE, so I don't know anything about it. Maybe you have one already for your home automation, otherwise, setting that up is left as the proverbial exercise to the reader.

The MQTT topic is "home/UPBtraffic" and can be changed in the ESP32UPBsecrets.h if that annoys you.