Large-scale VDI deployment management with Active Directory and domains

Thank you so much for your response. I really appreciate your feedback.

We are a Startup data center targeting Mid-size organization from 5-50 users to mostly external companies.

1. We want to provide VDI, M365, Mail, Onedrive, Azure backup, Azure Storage, Lighthouse and all Microsoft services through our Account.

2. VDI's will be hosted at our data center as each organization have different requirement viz, accounting, designing, drafting, documenting, high graphics, etc

3. We want each organization to be separate, but controlled by our domain.

4. There can be more than 300 organization with 5-50 users under each organization.

5. Customer Billing will be done, under our company, as we are providing different services to different organizations.

6. We will have our own AD, DNS, DCHP for on premises IT infrastructure.

7. We plan to sync AD to Azure AD via AD Connect.

8. Important concern is, 'AAA' organization shouldn't be able to communicate with 'BBB' organization.
Neither, organization should be able to see / view organization under our domain. e.g aaa.aaa.com shouldn't be able to communicate / view bbb.aaa.com or ccc.aaa.com... can this be done by GPO or ?

9. Should we consider sub-domain topology or any other is suggested ?

I look forward to your feedback

Best Regards