How to run Strapi in DEV mode with forever?

Not sure how forever works, but as an alternative, you can try pm2.

Create a file ecosystem.config.js in the project’s root directory with content:

module.exports = {
  apps: [
    {
      name: 'you-app-name',
      script: 'npm',
      args: 'develop',
      env: {
        NODE_ENV: 'development',
      },
      exp_backoff_restart_delay: 100,
    },
  ],
};

Now you can run the app with the command:
pm2 start ecosystem.config.js