GALSync and Office365

Hi

I would like to implement MIM GALSync across three Exchange Organizations. One of them uses Office365 (AzureAD and ADDS are synchronized using AzureAD Connect). But they don’t have an Exchange Server on-premises anymore. From my perspective the only way to establish proper GALSync is to deploy Exchange 2013 CAS Server on-prem to be able to provision mail-enabled contacts. Is there a more elegant way?

Another question. What would be the best way for GALSync if an Organization has Cloud-only Identities (No Sync between AzureAD and ADDS)?

Thanks for any suggestions best regards

Pirmin