You don’t want anything to do with the “system broker” stuff, when you are already using an independent broker (Mosquitto).
There are a couple of hundred posts about MQTT, I’m sure some of them cover establishing a broker connection with PaperUI. Do you really want to give IP, or should you be using localhost?
A large proportion of those posts also cover configuring for Tasmota.
This is mostly using xxx.things files, but the process is the same when using PaperUI
