Arduino Uno MQTT over USB

total scripting noob here. I, with the help of chatgpt, have been trying to publish data from my LM35 on Arduino Uno over MQTT to monitor the temps in my attic but without any luck.
Currently my setup consists of Windows Client PC, my Arduino Uno R3 and the LM35 (no esp or ethernetshield).
The Arduino has to send the data from the LM35 to the Windows Client over USB on Port COM3 which is supposed to act as the Broker.
I cannot seem to get it to output anything really. When i try to run the sketch, i always get scrambled gibberish (see attached image)
Gibberish output
When i publish something myself over the command line the data shows up on the sub side

I have tried a bunch of things but here is my current sketch, if anyone could help that would be awsome!

#include <OneWire.h>
#include <DallasTemperature.h>
#include <PubSubClient.h>

const char* mqtt_server = "10.6.53.4";
const int mqtt_port = 1883;
const char* mqtt_topic = "temp/attic";
const int oneWireBus = A0;

OneWire oneWire(oneWireBus);
DallasTemperature sensors(&oneWire);

PubSubClient mqttClient;

void setup() {
  Serial.begin(9600);
  sensors.begin();

  mqttClient.setServer(mqtt_server, mqtt_port);
  while (!mqttClient.connected()) {
    Serial.println("Connecting to MQTT broker...");
    if (mqttClient.connect("arduino_client")) {
      Serial.println("Connected to MQTT broker");
    } else {
      Serial.print("Failed with state ");
      Serial.print(mqttClient.state());
      delay(2000);
    }
  }
}

void loop() {
  sensors.requestTemperatures();
  float temperature = sensors.getTempCByIndex(0);
  
  char payload[6];
  dtostrf(temperature, 5, 2, payload);
  
  String topic = "temp/attic";

  if (!mqttClient.connected()) {
    Serial.print("Connecting to MQTT broker...");
    if (mqttClient.connect("ArduinoUnoClient")) {
      Serial.println("connected");
    } else {
      Serial.print("failed with state ");
      Serial.println(mqttClient.state());
      delay(2000);
      return;
    }
  }

  Serial.println("Connected to MQTT broker");
  Serial.println("Publishing temperature data...");
  Serial.print("Topic: ");
  Serial.println(topic);
  Serial.print("Payload: ");
  Serial.println(payload);

  mqttClient.publish(topic.c_str(), payload);

  delay(5000);
}

Unless your Arduino is acting as a USB-Ethernet adaptor this will NOT work. The PubSub client needs an active TCP stack to work, it can not just send "MQTT Packets" over USB.

While technically possible to send MQTT packets between 2 devices over nearly any 2 way connection, you would have to write the whole client side and the whole broker/usb2broker bridge side from scratch yourself.

A much better option here is to just remove ALL the MQTT code from the Arduino code and write the sensor readings to the serial out and write a short Python script to read those in and then publish them to a MQTT broker.

Take a look at https://github.com/vortex314/serial2mqtt, which stated goal is to "offer MQTT publisher/subscriber functionality to all small micro controllers. Those with just a UART or USB interface."

Another, more general option, could be https://github.com/mycontroller-org/2mqtt: "2mqtt is MQTT bridge. You can convert the serial, ethernet to MQTT"

I have not used any of them, so no guarantees.