Is there any possibility to do a tag or something more official in the ProjeQtOr SVN?

You are right, it should be a good practice.
On another hand the deployed zip file can also do the job : it includes all the code of the version, so there is not real need to go to the SVN reporitory.
But I'll think aboput tagging deployed version.

Also, do you consider to migrate from SVN to Git (like using GitHub) to make it easier from contributor to submit patches?

No.
Direct committing from community is not expected to become easier.
I really want to preserve today way of working, centralizing patches : this way, I can control what comes in the code, and I sometimes have to refuse some too much specific implementation or bad quality code.