Sails.js

Your tech stack is solid for building a real-time messaging project.

React and React Native are excellent choices for the frontend, especially if you want to have both web and mobile versions of your application share code.

ExpressJS is an unopinionated framework that affords you the flexibility to use it's features at your term, which is a good start. However, I would recommend you explore Sails.js as well. Sails.js is built on top of Express.js and it provides additional features out of the box, especially the Websocket integration that your project requires.

Don't forget to set up Graphql codegen, this would improve your dev experience (Add Typescript, if you can too).

I don't know much about databases but you might want to consider using NO-SQL. I used Firebase real-time db and aws dynamo db on a few of my personal projects and I love they're easy to work with and offer more flexibility for a chat application.

We have an existing (Apis only) Rails backend, that by default follows the MVC pattern, (at peaks of 700 requests a second). I am tasked with making the same (read-heavy) application in any JavaScript framework. I was advised to follow the MVC structure. So I am considering these 3 ( Sails.js, LoopBack, NestJS). I get that sails is closest to rails, but that's not particularly a priority.

For a small mobile app that uses mostly localization, to display Google Maps with hotspots nearby, which Cross-Platform would you prefer and why? It will make requests to a Sails.js app.