Unable to combine Area of Site permissions and Page Type Default permissions

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In my site I have configured permissions according to section. For example, Group A, can edit Section A pages, Group B can edit Section B pages etc.
This uses Area of Site permissions, i.e. Section A page has manual permissions and all child pages inherit this pages permissions. All good and working.

Next, my site's Page Types relate to variations on the page layout. Within each Page Type I want to restrict the types of Block a content editor can add to particular Areas, e.g. in a sidebar navigation limit the Blocks to AutoNav and PageList only. This can be achieved in Page Type Defaults by configuring permission on the Area.

Now, the problem I have comes when trying apply the Page Type permissions to the existing Area of Site permissions. It seems it can't be done, as the permissions for a page can only be Area of Site or Page Type Defaults (or Manual). It's not possible to have a combination.
Okay, why not configure the page edit permissions on the Page Type Defaults? Well, because my Page Types can be used across different sections of the website, and I want to restrict page editing for user groups according to section.

In the end I had to configure Area permissions on the parent page for each section, so that child pages inherit the Area permissions. The problem with this is that it takes a long time to configure and will be time consuming to manage.

Is there a better way to achieve this kind of permissions configuration in Concrete5?

Appreciate thoughts and solutions on this.

(version 5.6.2.1, advanced permissions)

adaptive