I have worked with Django, Flask, Laravel 5, and Node.js.
Based on my experience Node.js is your solution.
It's easy, reliable, developer friendly, suitable for REST API.
Due to its asynchronous nature, you don't have to worry about managing threads as we have to take that into consideration while working with other frameworks.
Also, it is Javascript. So less learning curve if you are coming from JAVA background( As you mentioned that you are mobile developer, assuming that you must be creating native apps).
I have created a basic boilerplate code for creating REST API's in Node.js using express and mongoose for MongoDB.
As your database is postrgres you might have to use different ORM like sequelize or something else.
Here are docs for sequelize:
I am not saying that it is the best way to create REST APIs. I am saying it is the easiest one I have used.
Also, if your target is only to build REST API, then you can choose any of the following Node.js frameworks:
My personal preference is Express.