Restrict activities access on the account

Requirement

My client has multiple Business Units and all BUs need Read Only access to ALL accounts but these BUs are competitive in nature and they should not be able to view each other activities. I understand if they have access to account they can see all activities but I want to restrict the visibility of all activites tasks, events and emails based on the BUs.

I can only create 2 restriction rules per object, therefore, can't use all the BUs to restrict activities. Also can I restrict visibility of emails using restrictions rules?

What other solutions I can provide my client to meet this requirement?