Hi @Russell
Question 1: Is there a way of removing the old server without grabbing a load of extra licences?
Unfortunately , there is no way , Azure AD premium license is required as mentioned in Microsoft documentation
Question 2: I encountered this whilst planning and preparing our migration from on-site Exchange to Exchange online. Do I need access to those featured to progress? Sync Errors seems like it would be important to check out.
Azure AD Connect Health is recommended to ensure synchronization monitoring but not required for your migration to Exchange online.
To get more details about Azure AD Connect Health please read the article below:
What is Azure AD Connect Health
Please don't forget to mark helpful answer as accepted