How Do You Handle A Company Wanting To Split Itself Into Multiple Companies With A Shared Services It

Currently company is a single entity. Hold Co LLC dba Acme Services. Microsoft 365 tenant, SSO, all licensing for all SaaS services are all in Acme Services name with a firstname.lastname@acmeservices.com UPN format. Things are working fine.

The powers that be want a form a new corporate structure that would see a new holding company structure that would transition a lot of the operations like IT, HR, Finance, etc. into a shared services model.

Each revenue generating department (of which there are 4) would break out into 4 separate companies, with the 5th company being the shared services for everything. I'm told that each company would eventually have its own email domain.

I'm told that some people who work for one of the 4 revenue companies may only do work/have access to files and SaaS platforms for that company only, but others might need to access files for some of the other companies.

We're a Microsoft 365 shop for IdP (with OneDrive for personal files) and Egnyte for cloud file services (department data). No on-prem anything, 100% cloud.

I'm a bit out of my depth on how exactly this would all work from an IdP perspective. My executive team is hoping that since each company is separate, that we can split our Microsoft licensing up so that we reset the 300 user limits for business premium for each company ... so 300, 300, 300, 300 basically because they would be different tenants.

I'm personally of the opinion to have the 'holding' company be the tenant, make all the other email domains aliases and just eat the cost of hundreds of E3 licenses after the first 300 business premium limit is reached.

This way, their UPN for SSO might be jsmith@holdingcompany.com, but that's not their email address for actually sending and receiving email. For email they might use jsmith@company1.com, or jsmith@company2.com, etc.

I'm mainly looking for concepts and ideas from people that have done similar things and help me understand my options and gotchas so I can do my own research. Just looking for some sage advice on the best approach for something like this.