How should I structure this code to run using .then in NodeJS and Express?

Question

exports.postSignup = async (req, res, next) => {
  const firstName = req.body.firstName;
  const lastName = req.body.lastName;
  const email = req.body.email;
  const password = req.body.password;

  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    console.log(errors.array());
    return res.status(422).render('auth/signup', {
      path: 'signup',
      pageTitle: 'Signup',
      errorMessage: errors.array()[0].msg,
      oldInput: {
        firstName: firstName,
        lastName: lastName,
        email: email,
        password: password,
        confirmPassword: req.body.confirmPassword
      },
      validationErrors: errors.array()
    });
  }

  bcrypt
    .hash(password, 12)
    .then(hashedPassword => {
      const user = new User({
        firstName: firstName,
        lastName: lastName,
        email: email,
        password: hashedPassword
      });
      return user.save();
    })
    .then(result => {

    if (req.session.cart) {
      const cart = await new Cart(req.session.cart);
      cart.save = req.user._id;
      await cart.save();
    }
      res.redirect('/login');
      const msg = {
        to: email, // recipient email
        from: { name: 'New User', email: '[email protected]' },
        templateId: process.env.WELCOME_TEMPLATE_ID
      }
      return sgMail
        .send(msg)
        .then(() => {
          console.log('Email sent')
        })
        .catch((error) => {
          console.error(error)
        })
    })
    .catch(err => {
      const error = new Error(err);
      error.httpStatusCode = 500;
      return next(error);
    });
};

An anonymous user can add items to cart. When they hit checkout, they're redirected to signup first before making a purchase. Now, I want to be able to check if the req has a session cart so I can register the user, then save that session cart in my Cart model so the cart session is lost. The code below is what I have so far. However, after a user signs up and try to checkout it seems the cart wasn't saved. I think there's a way to restructure the code but can't seem to get it. Any help will be appreciated!