Looks mostly okay, as a project package however I think I saw an issue somewhere (that others mentioned) about the copy command not being able to copy outside of the legacy folder (ie the Symfony Extension), there is a post process command (see the docs.suitecrm.com) where you can add a php script to run after processing, I would suggest you try copying your Symfony module into place manually with a PHP script rather than the manifest.php installer.
I havent actually used the installer with Suite8 apps (we are developing here with git and applying the files directly).
The actual code looks okay though, have you tried putting them in place on a stock Suite8 install?
Regards
Mark