AD domain segregation and sequence

Hi Team, 

We are working on AD separation project wherein we will going to have new domain setup and then migrate O365 services (user mailboxes and SharePoint ) for few users from existing tenant to new O365 tenant. Messaging environment is in hybrid with on premise exchange in existing domain. 

Tools we are considering are : ADMT and  BitTitan for Tenant to Tenant. Though I have below draft but identified few below GAPS . Need inputs .  

I understand the below high level steps : 

1. setup new domain . Migrate users from existing to new domain . 

2. Setup Azure AD connect and synch users to new O365 tenant

3. Migrate O365 user mailboxes from existing tenant to new O365 Tenant. 

GAP - Since mailbox migration will take sometime to move to new tenant. What will happen to email access during this time?  

How do we deal with free/busy between the scenario between users in 2 seprate domins with seprate O365 tenant? 

4. Migrate SP to new O365 SP 

Objective is to avoid disruption to users and making migration error free. 

Any pointers/or if have any document /article link from MS that would talk about the right sequencing for such migrations will help.

Regards