What to choose for the project as a DBMS, MS SQL Server or something else?

Question

Java

I have a project like diary.ru for college only.
List of students, electronic journal, tardiness journal, online testing and much more.
covers almost all college work.
I wrote it in both ASP.NET MVC and PHP. (ASP.NET MVC did not like the fact that it is strictly related to table names, for example, the Kontingent table should be named as "Kontingents" otherwise it stupidly did not see the table. Since there was already a data base that I wrote as a desktop several years ago.)
The PHP version is currently in use. As a MS SQL SERVER database.
At the moment, there are more than 2 million records in the database. The volume of the database file is about 1 GB.
It costs 2 servers:
1. fronted is NGINX, backend is Apache + PHP application (Server is weaker: 1 cpu, 4 RAM)
2. WinServer16. It runs AD and MS SQL Server (base for my application) (Server: 2 cpu, 16 RAM)
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In general, now the application is working fine, both at idle and under load.
But I was born to rewrite the program for Java. As the project grows and it becomes more and more difficult to maintain it. Now I think what is more correct, to use another DBMS or leave SQL Server? I thought to transfer AD to the first server as a separate controller and 4 RAM would be enough for him. and leave the normal 2nd server for the application under Java.
The link will be like this: Nginx-> Tomcat and DBMS (I don’t know which one yet)

Answer 1

https://www.postgresql.org/