Retiring our hybrid Exchange Server

 It looks like we may wait to retire the on-prem Exchange Server altogether, leaving a scaled down server for now to maintain attributes that can't be maintained online.  We can't convert the mailboxes totally to online since we still need the Active Directory accounts for access to other applications via AD security groups.  We are looking at converting Distribution Lists to Office 365 Groups to remove a level of dependency, but are still working through Office 365 Groups governance (different topic).  We really do want to get rid of every possible extra server to reduce the maintenance load on our staff - one of the reasons we went to Office 365.