Hello @biterbit
Thank you for your post.
I have read the entire case scenario stated previously, I would think of Management groups as a posible option for this task.
For instance, this works in a hierarchy level or group and on which each group will have its on permissions as well as among others.
Please refer to the link below:
https://learn.microsoft.com/en-us/azure/governance/management-groups/overview
A great image that you shall find on it is below.
I hope this was in someway useful for this management hierarchy request.
Cheers,
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Your welcome!