Migrating from source Hybrid environment to target Hybrid environment solution?

Asking the community for best solution on the current migration scenario. 

Source Environment:

Exchange 2013 cu18 Hybrid infrastructure with all mailboxes residing on-premises. 

- Office 365 EOP is used and inbound mail flow works from Public -> Mimecast -> EOP -> To On-premises mailboxes

- Primary SMTP address is smain.com

Target Environment:

Exchange 2013 Hybrid Infrastructure with all mailboxes residing in Exchange Online

Goal

Working to migrate all mailboxes from Source on-prem to Target O365

Problem

Source environments domain of smain.com cannot be added as an accepted domain in Target O365 due to the Source O365 having smain.com as an accepted domain.

Calendar syncs for co-existence cannot be created as calendar sync jobs will try to match source on-premises primary SMTP value to a mailbox user in target O365.

Question

What are the best architectural solutions to try and maintain a co-existence migration with free-busy?