Hi webdrifter:
I know absolutely nothing about Symfony, so any meaningful comparison between that CMS and MODx will have to come from someone else.
Like you, I know "some PHP" (just about enough to be dangerous

). MODx has made it easy for me to utilize my PHP skills in building some reasonably complicated functionality into my sites. Unlike the Big 3 (WP, Drupal, and Joomla) and similar systems, MODx doesn’t insist on squeezing you into its mold when you’re trying to do something new. Instead it acts more like an intuitive framework, keeping useful tools out of your way but within your reach. This is why the devs like to call it a "content management
framework" instead of a "content management
system."
It’s hard to address your particular situation from what you have shared about it, but you may at least be sure that developing complex interactive websites with lots of complex functionality is the specialty of MODx. I cannot objectively say that there is no system better - since I have never used another comparable system myself - but I can say that MODx absolutely excels.
If you are not good at developing graphical front-end stuff, I am afraid there is no tool that will be of much help to you in that area. Using MODx could help indirectly, however, because another of its great strengths is its total separation of frontend from backend, HTML from PHP. If you end up working with someone who is good at design, or using premade templates, you will find that it’s much easier in MODx than it would be in, say, Joomla.
Hope this helps clear things up a bit.