I arrived at a similar approach for my product documentation, with main sections:

-Tutorial/Quick start

-Reference (topic based)

-How-to guides (task based)

With an extra section:

-Expert tips (various things the product can do that aren't obvious or easy to discover)

It does mean that there is a certain amount of repetition between sections, but I don't think that is a big problem. You just have to be conscientious when updating things.

In my experience, the key thing about documentation is to write it as you are creating the product. Do not leave it all unti the end!