Managing large volume of content in tree hierarchy
Good strategy is to utilize Workbench and create sections from your menu. 

So you would have your main menu, lets say 3-4 levels deep and mimic those in Taxonomy. Its easier to create taxonomy first as there is a module that will create menu from taxonomy. 

So you create your taxonomy that has your structure going 3-4 levels deep. Then create a menu from it. 

Install Workbench and Workbench Access and Moderation modules. 

Configure Access to use Taxonomy for section assignment and then configure URL parameters to pull from Taxonomy as this will take care of your URL and breadcrumbs. Use Custom Breadcrumb module to generate breadcrumbs if you need them. 

Then on each content type add a new filed for Sections which will pull from your taxonomy. So when adding content you select where in the tree it should live. 

For example you are adding "Help" node which needs to live in About Us > Services  and About Us and Services are in the menu. Help does not need to be in menu but needs to follow the About Us > Services > Help path. 

By using Workbench modules you can easily achieve that. In a sense your taxonomy acts as buckets into which content is placed. This drives URL, Breadcrumbs as well as left or right nav menu if you need one using Menu Block. 

So to summarize the steps: 


Create taxonomy of the items you want to be in menu. 
Create menu from taxonomy using Taxonomy 2 Menu module. 
Install Workbench suite of modules. 
Configure workbench access to pull from Taxonomy. 
Adjust content types to add a fields for Taxonomy selection, make it only 1 as you don't want multiple paths for nodes. 
Configure URL Patterns to pull from taxonomy parents. 
Install and configure Custom Breadcrumbs to also use URL path. 
Install and configure Menu Block to use URL path. 
Start adding content and assigning sections. 


Additional benefits of this approach is that you can also restrict which roles or users can edit content in each section so you can have HR only have access to edit content in their part of the site.