A cloud-based service included in Microsoft 365, delivering scalable messaging and collaboration features with simplified management and automatic updates.
We've been asking for Graph API endpoints for Exchange management for years now, to no avail. There are some operations exposed via REST endpoints, as you mentioned below, however calling those outside of the PowerShell module is not officially supported. That said, if you don't care about the "supported" bit, you can certainly create a solution with no PowerShell dependencies.