What technology to choose to create a backend for an android application?

Question

Android

Hello.
Developed a mobile application. Now it's time to connect it to the data storage backend. I looked in the direction of Firebase and the like, they do not suit me because with my data volumes I will have to pay a lot. It was decided to develop the backend on our own to suit our needs, since time is running out.
You need to choose some kind of technology for development. Previously, I had nothing to do with the backend, except for a small study of RoR at the university.
Please advise any options. I can write quite well in Java/Kotlin. I also know a little about Ruby/RoR. If anything, I'm ready to learn a new language/framework.
What are the options for getting data from the backend?
Requirements:

• User registration via mobile application
  
• Separation of rights for registered users (you need to make three types of accounts with different levels of access to data)
  
• Storing user files (most likely to be Postgresql)
  
• ORM for working with the database
  
• Possibly an email push that is triggered from a mobile app
  

PS
Thanks to everyone who replied. I decided to wait with the development of the backend and use Firebase for now.

Answer 1

they do not suit me because with my data volumes I will have to pay a lot

a lot is how much?
Do you already have users, at least five thousand? if not, then don’t strain, make it possible to quickly switch to something else, spare it in architecture, and don’t worry, for most young projects, aws or frirebase will come out cheaper

Answer 2

Use the technology - what you know best.
Know Java well - write in Java. Everything you need for a specialized backend is for Java. Libraries, etc.
You can simply do this in the logic of your backend.
ORM for working with the database
Choose:
https://dzone.com/articles/best-java-orm-framework...

Answer 3

yes, anything, in fact,
as an option, you can use python (django + restframework or flask)
PS I would not touch RoR already