Assigning admin roles:

There is different ways to achieve BUs delegation:
o   Split to multiple tenants (other impacts)
o   Use a packaged third party offer (for example
It should also be possible to use group management provided by Azure AD (Azure AD Premium subscription required), but that need to be confirmed in term of feasibility and supportability:
Azure AD Delegated Group Management: Feature walk through


Please note that Azure AD Premium currently offers (in Preview)  a solution about delegation: