I want to know if there is any mistake in my script, or something that I
could add to it, in order to improving my results.
You have declared the "MyModel" class twice in your script. The first
declaration only adds symmetry restraints (which look fine) and the
second only renames chains. The second declaration will simply override
the first one, so your symmetry restraints won't be applied. Just remove
the second "class MyModel(automodel):" line from your script and it
should work.