index.js

const nodemailer = require('nodemailer');

const key = require('./key.json');

// Change this to one of your email addresses in the organisation
const YOUR_EMAIL_ADDRESS = 'info@your_company.com';
// Change this to the receiver to the mail
const SEND_TO = 'receiver@other_company.com'

async function start() {
  const transporter = nodemailer.createTransport({
    host: 'smtp.gmail.com',
    port: 465,
    secure: true,
    auth: {
      type: 'OAuth2',
      user: YOUR_EMAIL_ADDRESS,
      serviceClient: key.client_id,
      privateKey: key.private_key,
    },
  });
  try {
    await transporter.verify();
    await transporter.sendMail({
      from: YOUR_EMAIL_ADDRESS,
      to: SEND_TO,
      subject: 'John Doe opens new farm YOU GOTTA SEE IT',
      text: 'It is beautiful.',
    });
  } catch (err) {
    console.error(err);
  }
}

start();

Yes, thanks @Roverr! Really invaluable.

Still working as of April 2021 - just with the same observation that @sabasm made regarding the change in Google console.
https://medium.com/@imre_7961/nodemailer-with-g-suite-oauth2-4c86049f778a

Thank you very much for guiding me through the Google maze :) All working perfectly!