Manage customized access rights for subfolders while inheriting access rights from top feasible?

Hi,

short question on sharepoint, sure, very easy for you.

We are currently building a sharepoint with the following characteristics:

Folder structure:
Hierarchical folder structure, i.e. any folder can have multiple subfolders.

Access rights:
Access rights shall be provided for user groups to all folders. Any user group that has access to a folder, shall always have also access to the folder's subfolders. However, any user group having access to a subfolder shall not have access to the folders above. (i.e. top guys see everything, lower level people only see there stuff)

Implementation via inheritance:
As far as I now understood, inheritance is a very good means to set up the proposed structure of folders and access rights. The step-wise approach would look the following way:
1. Create folders
2. Create user groups
3. Add user groups to folders (i.e. start at top, add user groups to top level, break inheritance to subfolders, add user groups to subfolders, break inheritance to next level ... and so on)

Issue:
If I would like to add one further user group at the top level of the folder structure, I would have to add it either to each folder individually (in our case we will have 100s of different folders with different access rights) or we would have to start step 3 from above all over again.

Both solutions seem very work intensive and I wonder, whether there is another solution?
I would expect some kind of function in sharepoint where I can inherit user rights from the top without deleting individual user rights in the subfolders (i.e. a function that checks whether there is any user group that has access to a folder, but not to its subfolders despite having inheritance; but at the same time not delete groups that shall only have access to the subfolders, but not to the folder above).

I would highly appreciate any advice on how to solve this issue without using the above mentioned work-intensive options.

Thank you very much in advance!

Best regards,
Benjamin