Give web app permanent control of google email - express

Question

I recently got a domain from Google Domain and it provides professional emails for the domain. So, I made a support#domain email to send and receive emails. Right now, I built a web app that will use this email to send email-verification to new users who sign up.
I got the code working, however, I used OAuth2.0 from Google to allow the web app to sign into the support email. This causes the Access token to expire and forces me to into my .env file and replace BOTH tokens. How tedious! If I were to publish this web app, I can't just go into my heroku vars and replace the tokens every hour. Thats extremely impractical.
I looked into service accounts by google, but they seem to STILL need OAuth anyways as a 3 legged OAuth. As I am using Express.js, I wanted to know if there was a way to set the tokens and be done with it permanently. Essentially giving the web app permanent control of the google account. What do I do? And what do I use? All advice is greatly appreciated.
Code for sending email verification:
const nodemailer = require("nodemailer")
const dotenv = require("dotenv")
const {generateVerifyToken} = require("./auth")
const { print } = require("#AlecRuin/color-logger")
dotenv.config()
let transporter = nodemailer.createTransport({
service:"gmail",
auth: {
type: 'OAuth2',
user: process.env.EMAIL,
clientId: process.env.CLIENT_ID,
clientSecret: process.env.CLIENT_SECRET,
refreshToken: process.env.REFRESH_TOKEN,
accessToken: process.env.ACCESS_TOKEN
}
})
module.exports= async function(email){
try {
let mailOptions={
from: "support#domain.us",
to:email,
subject:"Verify your email",
text:`
Thank you for signing up with redacted!!
To verify your email (and make sure you're not some kind of annoying bot), simply follow this link:
${process.env.NODE_ENV === "production"?"www.redacted.com":"localhost:3001"}/api/user/verify/?token=${generateVerifyToken(email)}
`
}
print("sending email",new Error,{isClient:false})
transporter.sendMail(mailOptions,(err,data)=>{
if (err){
console.error(err);
return err
}else{
print("Email sent!",new Error,{isClient:false})
return true
}
})
} catch (error) {
print(error,new Error, {isClient:true,severity:4})
return error
}
}