If you create an environment variable called DATABASE_URL with the “Internal Database URL” from the Render Postgres instance, you should be able to connect with just:
const sequelize = new Sequelize(process.env.DATABASE_URL)
As covered in their docs: https://sequelize.org/docs/v6/getting-started/#connecting-to-a-database.
Once you get the connection sorted, you can expand on what other configuration you want.
Alan