Deserialization error arduino uno and nodemcu esp8266 to Thingspeak

I am currently sending temperature and humidity readings to a Thingspeak channel via a nodemcu esp8266 wifi module using serial communication between the arduino and esp. I have set the time between sending data to Thingspeak to 30s, however the time stamps I receive on my Thingspeak channel are very inconsistent even though the data has been supposedly sent (ranging from 3 minutes to 15 minutes). I suspect this could be due to some timing error as I receive many deserialization errors when I convert the Json string (deserialize it on the NodeMCU side). Any help would be appreciated! Note, this is only a part of the project, there are multiple relays added, thus using the nodemcu on its own would be insufficient. I have also added some images, which shows the outputs.

Arduino Uno code:

//Arduino side code
#include <DHT.h>

#include <avr/wdt.h>                //Watchdog Timer

#include <SoftwareSerial.h>         //Arduino to NodeMCU Lib
#include <ArduinoJson.h>

SoftwareSerial nodemcu(5, 6);       //Initialise Arduino to NodeMCU (5=Rx & 6=Tx)

//Initialisation of DHT22 Sensor
#define DHTPIN 4
DHT dht(DHTPIN, DHT22);
float tempC;
float humidity;
float VPD;

void setup() {
  Serial.begin(9600);
  dht.begin();
  nodemcu.begin(9600);
  delay(2000);

  wdt_enable(WDTO_4S);              //Enable Watchdog timer

  Serial.println("Program started");
}

void loop() {
  StaticJsonDocument<1000> doc;

  dht22_func();                          //Obtain Temp and Hum data

  doc["humidity"] = humidity;            //Assign collected data to JSON Object
  doc["temperature"] = tempC;
  doc["VPD"] = VPD;

  serializeJson(doc, nodemcu);           //Send data to NodeMCU

  wdt_reset();                           //Reset for WDT
}

void dht22_func() {

  humidity = dht.readHumidity();
  tempC = dht.readTemperature();
  Serial.print("Humidity: ");
  Serial.println(humidity);
  Serial.print("Temperature: ");
  Serial.println(tempC);

  if (isnan(humidity) || isnan(tempC) || humidity == 0.0 || tempC == 0.0) {
    while (1) {}
  }
}

NodeMCU code:

#include <SoftwareSerial.h>
#include <ArduinoJson.h>

#include <ESP8266WiFi.h>
#include <ThingSpeak.h>    

SoftwareSerial nodemcu(D6, D5);                 //D6 = Rx & D5 = Tx

unsigned long previousMillis = 0;
unsigned long currentMillis;
const unsigned long period = 10000;             //Upload interval in ms

float tempC;
float humidity;
float VPD;

String apiKey = "";             //Write API key

unsigned long channelID = ;              //Thingspeak channel ID
const char *ssid =  "";             //Local network ssid
const char *pass =  "";                 //Local network password
const char* server = "api.thingspeak.com";      //Thingspeak server

boolean state1 = false;                         //Boolean variables
boolean state2 = false;

WiFiClient client;

void setup() {
  Serial.begin(9600);
  nodemcu.begin(9600);
  ThingSpeak.begin(client);
  while (!Serial) continue;
  delay(2000);

  Serial.println("Program Started");

  wifi_connect();                                      //Establish wifi connection
}

void loop() {
  currentMillis = millis();                                  //Get current time

  if ((currentMillis - previousMillis >= period)) {

    if (WiFi.status() != WL_CONNECTED) {                    //Check if connected to wifi, else 
connect
      wifi_connect();
}
else {
  Serial.println("Currently Connected to WiFi");
}

while (state1 == false) {                                //Connect to Thingspeak and push data

  while (state2 == false) {
    parseJsonObject();
  }
  
  cloud_connect();
}

state1 = false;
state2 = false;
Serial.println("Cloud Connect Succesful");
Serial.println("-----------------------------------------");
previousMillis = previousMillis + period;                     //Update counter   
  }
}

void wifi_connect() {
  Serial.println("Connecting to ");
  Serial.println(ssid);

  WiFi.begin(ssid, pass);

  while (WiFi.status() != WL_CONNECTED)
  {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.println("WiFi Connected");
}

void cloud_connect() {
  if (client.connect(server, 80)) {           //80 is the standard port number used for web access
    String postStr = apiKey;
    postStr += "&field1=";
    postStr += String(tempC);
    postStr += "&field2=";
    postStr += String(humidity);
    postStr += "\r\n\r\n";

    client.print("POST /update HTTP/1.1\n");
    client.print("Host: api.thingspeak.com\n");
    client.print("Connection: close\n");
    client.print("X-THINGSPEAKAPIKEY: " + apiKey + "\n");
    client.print("Content-Type: application/x-www-form-urlencoded\n");
    client.print("Content-Length: ");
    client.print(postStr.length());
    client.print("\n\n");
    client.print(postStr);

    Serial.print("Temperature: ");
    Serial.print(tempC);
    Serial.print(" || Humidity: ");
    Serial.println(humidiy);
    Serial.println("Sent to Thingspeak");

    //Change state to true to break while loop
  state1 = true;
  }
  client.stop();
}

void parseJsonObject() {
  //Parse JSON object from serial port
  StaticJsonDocument<1000> doc;
  DeserializationError error = deserializeJson(doc, nodemcu);

  // Test if parsing succeeds
  if (error) {
    Serial.println("deserializeJson error");
    return;
  }

  //Parse Json values and store in variable
  humidity = doc["humidity"];
  tempC = doc["temperature"];

  if (isnan(humidity) || isnan(tempC) || humidity == 0.0 || tempC == 0.0) {
    Serial.println("Invalid Sensor Readings");
    return;
  }

  state2 = true;
  Serial.println("JSON Object Recieved");
}