Node.js and ExpressJS is a good choice for backend, the stack makes it really easy to build a REST API.

I'm not exactly sure what you mean by using ExpressJS as an admin panel. Express is a lightweight backend framework for Node.js that lets you build your endpoints quickly and with minimal code. If you're looking for an admin panel to manage the data on your mobile app, check out BaseDash. It lets you view and edit data from your SQL database with the simplicity of Airtable.