If you aren't confident to build on top of what you've created so far there's not much hope you will do any better on a 3rd party application.
No matter what platform you go to, be it a framework or a platform like WP or Drupal, there is a large learning curve as well as a need to learn the fundamentals of programming to not create something horrible.
Also the notion that it will ever be the best it can be is pointless, it never will be. What you are looking for is an acceptable level, which it sounds like you aren't there yet.
If you had to go with a platform, I would recommend Drupal - it is intended to be more customizable and gives quite a bit of control over individual user access via roles. However Drupal is, like any other platform, limited by its own complexity, so if you need something very customized - custom code is always the way to go.
If you possibly have the budget, you could consider hiring a contractor with a lot of experience to work with you because it honestly sounds like you are in way over your head. Then again if this isn't mission critical - it could be a great learning experience if you decide to dig in and go at it.