Two-way sync between third-party XML export and SharePoint Online

Hi conwayjh 

did you need to update your Term store and update managed metadata?
Is it correct the needs?

Imho, you can define an Azure AD Application  and trust SharePoint Online for reading and write Metadata, writing an azure function in C# and SharePoint CSOM (example). In this way, with an application id and a self-signed cert, you are working with an s2s authentication. The Azure function could be scheduled.

I suggest you use SharePointPnPCoreOnline library, with a lot of useful extension methods

You can also consider if you don't need high reliability or others skills, Microsoft Flow, there is SQL Server connector that could trigger your azure function. Actually, there is not REST API for manage Terms or TermSet creation.

Cheers,

Federico